user8238: It is often stated particularly by Theravada Buddhism that Nibbana/Nirvana is unconditioned. In fact extensive metaphysical speculations have even been written about by esteemed monks.
http://www.accesstoinsight.org/lib/authors/thanissaro/resonance.html
Where is this actually stated in the canonical scripture? This doctrine seems particularly important and should have extensive discourse by the Buddha.
In fact, it would appears that this doctrine is in contradiction with the doctrine of dependent origination. And it would not be clear how the Four Noble Truths and the Eightfold Noble Path arrived at an "Unconditioned Nibana".

When this is, that is.
From the arising of this comes the arising of that.
When this isn't, that isn't.
From the cessation of this comes the cessation of that.

It was said that Venerable Sariputta upon hearing the dependent generation stanzas out of his profound wisdom immediately became a stream winner, knowing the way to end suffering.
I believe the reasoning is as follows:

Things that arise due to causes and conditions cease with the end of
those causes and conditions.
Suffering arise due to causes and conditions
Suffering therefore can be extinguish with the right causes and
conditions.
The right causes and conditions are the Practice of the Noble
Eightfold Path, which leads to Ethics, Concentration and Wisdom,
removing the causes for suffering.

But this mean that even the cessation of suffering Nibana itself is conditional!
I did some research on Sutta Central and found the following
https://suttacentral.net/en/sn43.12

“Bhikkhus, I will teach you the unconditioned and the path leading to
  the unconditioned. Listen to that….
“And what, bhikkhus, is the unconditioned? The destruction of lust,
  the destruction of hatred, the destruction of delusion: this is called
  the unconditioned.
“And what, bhikkhus, is the path leading to the unconditioned?
  Serenity: this is called the path leading to the unconditioned….
“Thus, bhikkhus, I have taught you the unconditioned and the path
  leading to the unconditioned…. This is our instruction to you.”
“Thus, bhikkhus, I have taught you the unconditioned and the path
  leading to the unconditioned. Whatever should be done, bhikkhus, by a
  compassionate teacher out of compassion for his disciples, desiring
  their welfare, that I have done for you. These are the feet of trees,
  bhikkhus, these are empty huts. Meditate, bhikkhus, do not be
  negligent, lest you regret it later. This is our instruction to you.”

However, there is no correspondent parallel text in the Chinese Agama, and the content is lacking substance having no message apart from Buddhist practice leading to Unconditionality, and hence can be suspected to be a Theravadin innovation and a latter doctrinal addition.
Likewise with
https://suttacentral.net/en/ud8.3

Thus I heard: At one time the Gracious One was dwelling near Sāvatthī,
  in Jeta’s Wood, at Anāthapiṇḍika’s monastery. Then at that time the
  Gracious One was instructing, rousing, enthusing, and cheering the
  monks with a Dhamma talk connected with Emancipation. Those monks,
  after making it their goal, applying their minds, considering it with
  all their mind, were listening to Dhamma with an attentive ear.
Then the Gracious One, having understood the significance of it, on
  that occasion uttered this exalted utterance:
“There is, monks, an unborn, unbecome, unmade, unconditioned. If,
  monks there were not that unborn, unbecome, unmade, unconditioned, you
  could not know an escape here from the born, become, made, and
  conditioned. But because there is an unborn, unbecome, unmade,
  unconditioned, therefore you do know an escape from the born, become,
  made, and conditioned.”

This sounds like an illogical tautology - circular reasoning supporting the unconditional Nibbana doctrine.
The fact that this would be said by someone as enlightened as the Buddha is extremely slim.
The main reason I can imagine this being used is the use of Nirvana (Cessation/Blown Out) as a negation, i.e. Nothing is Unconditioned, therefore Nirvana is Unconditioned.
I would appreciate it if anyone can provide canonical sources that explain the importance of unconditionality with respect to Nibbana and Nirvana. Particularly why is it even important and consistent with the rest of Buddhist teachings? Please feel free to point out where you think my own views might be mistaken.

user8157: The path is conditioned:

Monks, among all things conditioned, the Noble Eightfold Path is reckoned to be the best of them all. Those who have faith in the
  Noble Eightfold Path have faith in the best; and for those who have
  faith in the best, the best result will be theirs. AN 4:34

The destination is unconditioned:

Monks, among things conditioned and unconditioned, dispassion is reckoned to be the best of them all: the crushing of all infatuation,
  the removal of thirst, the uprooting of attachment, the cutting off of
  the round, the destruction of craving, dispassion,
  Nibbāna. Those who have faith in the Dhamma of dispassion have
  faith in the best; and for those who have faith in the best, the best
  result will be theirs. AN 4:34

There is, monks, an unborn, unbecome, unmade, unconditioned. If, monks there were not that unborn, unbecome, unmade, unconditioned, you
  could not know an escape here from the born, become, made, and
  conditioned. But because there is an unborn, unbecome, unmade,
  unconditioned, therefore you do know an escape from the born, become,
  made, and conditioned. Ud 8.3

For example, a fire burns conditioned by wood. The same fire is extinguished, by using water. Using water to extinguish the fire is conditoned. But what remains, namely, the absence of fire, is not conditioned by the water because it is not dependent upon the water. When the fire is extinguished, the subsequent evaporation or absence of the water is not the condition for the absence of fire. The fire had a cause (e.g. lightning striking a tree); the extinguishing of the fire had a cause (e.g. hosing with water) but the staying out of the fire is not conditioned by the water. It is the absence of fire that is Nibbana. 
The path removes what is hidden. But the path does not create or cause what is hidden. 
Similarly, cutting down a tree is dependent upon (conditioned by) an axe. But once tree is cut down, what remains is not dependent upon the axe. Thus what remains is 'unconditioned' since what remains has always been there but the tree covered what was always already there. 

Bhikkhus, this bhikkhu is called one whose cross-bar has been lifted, whose trench has been filled in, whose pillar has been uprooted, one
  who has no bolt, a noble one whose banner is lowered, whose burden is
  lowered, who is unfettered.
And how is the bhikkhu one whose cross-bar has been lifted? Here the bhikkhu has abandoned ignorance, has cut it off at the root, made it
  like a palm stump, done away with it, so that it is no longer subject
  to future arising. That is how the bhikkhu is one whose cross-bar has
  been lifted.
And how is the bhikkhu one whose trench has been filled in? Here the bhikkhu has abandoned the round of births that brings renewed being,
  has cut it off at the root…so that it is no longer subject to future
  arising. That is how the bhikkhu is one whose trench has been filled
  in.
And how is the bhikkhu one whose pillar has been uprooted? Here the bhikkhu has abandoned craving, has cut it off at the root…so that it
  is no longer subject to future arising. That is how the bhikkhu is one
  whose pillar has been uprooted.
And how is the bhikkhu one who has no bolt? Here the bhikkhu has abandoned the five lower fetters, has cut them off at the root…so that
  they are no longer subject to future arising. That is how the bhikkhu
  is one who has no bolt.
And how is the bhikkhu a noble one whose banner is lowered, whose burden is lowered, who is unfettered? Here a bhikkhu has abandoned the
  conceit ‘I am,’ has cut it off at the root so that it is no longer
  subject to future arising. That is how the bhikkhu is a noble one
  whose banner is lowered, whose burden is lowered, who is unfettered.
Bhikkhus, when the gods with Indra, with Brahmā and with Pajāpati seek a bhikkhu who is thus liberated in mind, they do not find
  anything of which they could say: ‘The consciousness of one thus gone
  is supported by this.’ Why is that? One thus gone, I say, is
  untraceable here and now.
MN 22


user254: Where is this actually stated in the canonical scripture?
After only a quick search I don't find many references other than the two you already mentioned (assuming you're only looking for suttas and not for example the Visuddhimagga).
There's another here: The Nibbāna-element.
There's also (the famous) Dhamapada verse 154 (which says visaṅ­khā­ra­ rather than asaṅkhata).
But this mean that even the cessation of suffering Nibanna itself is conditional!
Yes I think that's so: that cessation (i.e. transition from suffering to non-suffering) is conditional ... it's conditional on the removal of the fuel of suffering.
I think the understanding is that, after the condition for suffering have been removed, then the non-suffering (i.e. the non-arising of further suffering) is unconditioned.
For example if you look at an empty fireplace (empty of fuel), then I think you can say "that fire is never going to light again there ... because there is no fuel."
You could bring more fuel, I suppose (so you could say it's conditional on the not-bringing of more fuel), but apparently an Arhat has learned better than to want to do that (and I think might even be unable to do that: unable to see the non-desirable as desirable, for example)
As you say, though, maybe it's not an especially important assertion, and is metaphysical.
I think that there other places where "following the path" is given as a condition for arriving; but that, having arrived, those conditions no longer applicable; for example:

A raft, well-made,
  has been lashed together.
  Having crossed over,
  gone to the far shore,
  I've subdued the flood.
  No need for a raft
  is to be found:
      so if you want, rain-god,
      go ahead & rain.

A raft is also mentioned in MN 22:

Monks, I will teach you the Dhamma compared to a raft, for the purpose of crossing over, not for the purpose of holding onto.

Or the Bhikkhuni Sutta, which says that,

This body comes into being through [food, craving, and conceit]. And yet it is by relying on [food, craving, and conceit] that [food, craving, and conceit] are to be abandoned.

This sounds like an illogical tautology - circular reasoning supporting the unconditional Nibanna doctrine.
Logically, yes, that does seem to be begging the question.
I think it's worth asking, not whether it's a good logical proof for the existence of "the unconditioned", but whether it has an effective pedagogical result, e.g. whether it helps the students to understand emancipation from the conditioned. 
That reminds me of another fallacy, i.e. Zeno's paradoxes of motion which "prove" that you can never arrive somewhere.
Something like that paradox is addressed in the Brahmana Sutta (in that sutta I think that  "park" means "monastery").

8238: <p>It is often stated particularly by Theravada Buddhism that Nibbana/Nirvana is unconditioned. In fact extensive metaphysical speculations have even been written about by esteemed monks.</p>

<p><a href="http://www.accesstoinsight.org/lib/authors/thanissaro/resonance.html" rel="nofollow noreferrer">http://www.accesstoinsight.org/lib/authors/thanissaro/resonance.html</a></p>

<p>Where is this actually stated in the canonical scripture? This doctrine seems particularly important and should have extensive discourse by the Buddha.</p>

<p>In fact, it would appears that this doctrine is in contradiction with the doctrine of dependent origination. And it would not be clear how the Four Noble Truths and the Eightfold Noble Path arrived at an "Unconditioned Nibana".</p>

<blockquote>
  <p>When this is, that is.</p>
  
  <p>From the arising of this comes the arising of that.</p>
  
  <p>When this isn't, that isn't.</p>
  
  <p>From the cessation of this comes the cessation of that.</p>
</blockquote>

<p>It was said that Venerable Sariputta upon hearing the dependent generation stanzas out of his profound wisdom immediately became a stream winner, knowing the way to end suffering.</p>

<p>I believe the reasoning is as follows:</p>

<ul>
<li>Things that arise due to causes and conditions cease with the end of
those causes and conditions.</li>
<li>Suffering arise due to causes and conditions</li>
<li>Suffering therefore can be extinguish with the right causes and
conditions.</li>
<li>The right causes and conditions are the Practice of the Noble
Eightfold Path, which leads to Ethics, Concentration and Wisdom,
removing the causes for suffering.</li>
</ul>

<p>But this mean that even the cessation of suffering Nibana itself is conditional!</p>

<p>I did some research on Sutta Central and found the following</p>

<p><a href="https://suttacentral.net/en/sn43.12" rel="nofollow noreferrer">https://suttacentral.net/en/sn43.12</a></p>

<blockquote>
  <p>“Bhikkhus, I will teach you the unconditioned and the path leading to
  the unconditioned. Listen to that….</p>
  
  <p>“And what, bhikkhus, is the unconditioned? The destruction of lust,
  the destruction of hatred, the destruction of delusion: this is called
  the unconditioned.</p>
  
  <p>“And what, bhikkhus, is the path leading to the unconditioned?
  Serenity: this is called the path leading to the unconditioned….</p>
  
  <p>“Thus, bhikkhus, I have taught you the unconditioned and the path
  leading to the unconditioned…. This is our instruction to you.”</p>
  
  <p>“Thus, bhikkhus, I have taught you the unconditioned and the path
  leading to the unconditioned. Whatever should be done, bhikkhus, by a
  compassionate teacher out of compassion for his disciples, desiring
  their welfare, that I have done for you. These are the feet of trees,
  bhikkhus, these are empty huts. Meditate, bhikkhus, do not be
  negligent, lest you regret it later. This is our instruction to you.”</p>
</blockquote>

<p>However, there is no correspondent parallel text in the Chinese Agama, and the content is lacking substance having no message apart from Buddhist practice leading to Unconditionality, and hence can be suspected to be a Theravadin innovation and a latter doctrinal addition.</p>

<p>Likewise with</p>

<p><a href="https://suttacentral.net/en/ud8.3" rel="nofollow noreferrer">https://suttacentral.net/en/ud8.3</a></p>

<blockquote>
  <p>Thus I heard: At one time the Gracious One was dwelling near Sāvatthī,
  in Jeta’s Wood, at Anāthapiṇḍika’s monastery. Then at that time the
  Gracious One was instructing, rousing, enthusing, and cheering the
  monks with a Dhamma talk connected with Emancipation. Those monks,
  after making it their goal, applying their minds, considering it with
  all their mind, were listening to Dhamma with an attentive ear.</p>
  
  <p>Then the Gracious One, having understood the significance of it, on
  that occasion uttered this exalted utterance:</p>
  
  <p>“There is, monks, an unborn, unbecome, unmade, unconditioned. If,
  monks there were not that unborn, unbecome, unmade, unconditioned, you
  could not know an escape here from the born, become, made, and
  conditioned. But because there is an unborn, unbecome, unmade,
  unconditioned, therefore you do know an escape from the born, become,
  made, and conditioned.”</p>
</blockquote>

<p>This sounds like an illogical tautology - circular reasoning supporting the unconditional Nibbana doctrine.</p>

<p>The fact that this would be said by someone as enlightened as the Buddha is extremely slim.</p>

<p>The main reason I can imagine this being used is the use of Nirvana (Cessation/Blown Out) as a negation, i.e. Nothing is Unconditioned, therefore Nirvana is Unconditioned.</p>

<p>I would appreciate it if anyone can provide canonical sources that explain the importance of unconditionality with respect to Nibbana and Nirvana. Particularly why is it even important and consistent with the rest of Buddhist teachings? Please feel free to point out where you think my own views might be mistaken.</p>
 8157: <p>The path is conditioned:</p>

<blockquote>
  <p><em>Monks, among all things conditioned, the Noble Eightfold Path is reckoned to be the best of them all. Those who have faith in the
  Noble Eightfold Path have faith in the best; and for those who have
  faith in the best, the best result will be theirs. AN 4:34</em></p>
</blockquote>

<p>The destination is unconditioned:</p>

<blockquote>
  <p><em>Monks, among things conditioned and unconditioned, dispassion is reckoned to be the best of them all: the crushing of all infatuation,
  the removal of thirst, the uprooting of attachment, the cutting off of
  the round, the destruction of craving, dispassion,
  Nibbāna. Those who have faith in the Dhamma of dispassion have
  faith in the best; and for those who have faith in the best, the best
  result will be theirs. AN 4:34</em></p>
  
  <hr>
  
  <p><em>There is, monks, an unborn, unbecome, unmade, unconditioned. If, monks there were not that unborn, unbecome, unmade, unconditioned, you
  could not know an escape here from the born, become, made, and
  conditioned. But because there is an unborn, unbecome, unmade,
  unconditioned, therefore you do know an escape from the born, become,
  made, and conditioned. Ud 8.3</em></p>
</blockquote>

<p>For example, a fire burns <em>conditioned</em> by wood. The same fire is extinguished, by using water. Using water to extinguish the fire is conditoned. But what remains, namely, the absence of fire, is not conditioned by the water because it is <em>not dependent</em> upon the water. When the fire is extinguished, the subsequent evaporation or absence of the water is not the condition for the absence of fire. The fire had a cause (e.g. lightning striking a tree); the extinguishing of the fire had a cause (e.g. hosing with water) but the staying out of the fire is not conditioned by the water. It is the absence of fire that is Nibbana. </p>

<p>The path removes what is hidden. But the path does not create or cause what is hidden. </p>

<p>Similarly, cutting down a tree is dependent upon (conditioned by) an axe. But once tree is cut down, what remains is not dependent upon the axe. Thus what remains is 'unconditioned' since what remains has always been there but the tree covered what was always already there. </p>

<blockquote>
  <p><em>Bhikkhus, this bhikkhu is called one whose cross-bar has been lifted, whose trench has been filled in, whose pillar has been uprooted, one
  who has no bolt, a noble one whose banner is lowered, whose burden is
  lowered, who is unfettered.</em></p>
  
  <p><em>And how is the bhikkhu one whose cross-bar has been lifted? Here the bhikkhu has abandoned ignorance, has cut it off at the root, made it
  like a palm stump, done away with it, so that it is no longer subject
  to future arising. That is how the bhikkhu is one whose cross-bar has
  been lifted.</em></p>
  
  <p><em>And how is the bhikkhu one whose trench has been filled in? Here the bhikkhu has abandoned the round of births that brings renewed being,
  has cut it off at the root…so that it is no longer subject to future
  arising. That is how the bhikkhu is one whose trench has been filled
  in.</em></p>
  
  <p><em>And how is the bhikkhu one whose pillar has been uprooted? Here the bhikkhu has abandoned craving, has cut it off at the root…so that it
  is no longer subject to future arising. That is how the bhikkhu is one
  whose pillar has been uprooted.</em></p>
  
  <p><em>And how is the bhikkhu one who has no bolt? Here the bhikkhu has abandoned the five lower fetters, has cut them off at the root…so that
  they are no longer subject to future arising. That is how the bhikkhu
  is one who has no bolt.</em></p>
  
  <p><em>And how is the bhikkhu a noble one whose banner is lowered, whose burden is lowered, who is unfettered? Here a bhikkhu has abandoned the
  conceit ‘I am,’ has cut it off at the root so that it is no longer
  subject to future arising. That is how the bhikkhu is a noble one
  whose banner is lowered, whose burden is lowered, who is unfettered.</em></p>
  
  <p><em>Bhikkhus, when the gods with Indra, with Brahmā and with Pajāpati seek a bhikkhu who is thus liberated in mind, they do not find
  anything of which they could say: ‘The consciousness of one thus gone
  is supported by this.’ Why is that? One thus gone, I say, is
  untraceable here and now.</em></p>
  
  <p><em>MN 22</em></p>
</blockquote>
 254: <p><strong>Where is this actually stated in the canonical scripture?</strong></p>

<p>After only a quick search I don't find many references other than the two you already mentioned (assuming you're only looking for suttas and not for example the Visuddhimagga).</p>

<p>There's another here: <a href="https://suttacentral.net/en/iti44" rel="nofollow noreferrer">The Nibbāna-element</a>.</p>

<p>There's also (the famous) <a href="http://www.tipitaka.net/tipitaka/dhp/verseload.php?verse=153" rel="nofollow noreferrer">Dhamapada verse 154</a> (which says <em>visaṅ­khā­ra­</em> rather than <em>asaṅkhata</em>).</p>

<p><strong>But this mean that even the cessation of suffering Nibanna itself is conditional!</strong></p>

<p>Yes I think that's so: that cessation (i.e. transition from suffering to non-suffering) is conditional ... it's conditional on the removal of the fuel of suffering.</p>

<p>I think the understanding is that, after the condition for suffering have been removed, then the non-suffering (i.e. the non-arising of further suffering) is unconditioned.
For example if you look at an empty fireplace (empty of fuel), then I think you can say "that fire is <em>never</em> going to light again there ... because there is no fuel."
You could bring more fuel, I suppose (so you could say it's conditional on the not-bringing of more fuel), but apparently an Arhat has learned better than to want to do that (and I think might even be unable to do that: unable to see the non-desirable as desirable, for example)</p>

<p>As you say, though, maybe it's not an especially important assertion, and is metaphysical.</p>

<p>I think that there other places where "following the path" is given as a condition for arriving; but that, having arrived, those conditions no longer applicable; <a href="http://www.accesstoinsight.org/tipitaka/kn/snp/snp.1.02.than.html" rel="nofollow noreferrer">for example</a>:</p>

<blockquote>
  <p>A raft, well-made,<br>
  has been lashed together.<br>
  Having crossed over,<br>
  gone to the far shore,<br>
  I've subdued the flood.<br>
  No need for a raft<br>
  is to be found:<br>
      so if you want, rain-god,<br>
      go ahead &amp; rain.</p>
</blockquote>

<p>A raft is also mentioned in <a href="http://www.accesstoinsight.org/tipitaka/mn/mn.022.than.html" rel="nofollow noreferrer">MN 22</a>:</p>

<blockquote>
  <p>Monks, I will teach you the Dhamma compared to a raft, for the purpose of crossing over, not for the purpose of holding onto.</p>
</blockquote>

<p>Or the Bhikkhuni Sutta, which says that,</p>

<blockquote>
  <p>This body comes into being through [food, craving, and conceit]. And yet it is by relying on [food, craving, and conceit] that [food, craving, and conceit] are to be abandoned.</p>
</blockquote>

<p><strong>This sounds like an illogical tautology - circular reasoning supporting the unconditional Nibanna doctrine.</strong></p>

<p>Logically, yes, that does seem to be <a href="https://en.wikipedia.org/wiki/Begging_the_question" rel="nofollow noreferrer">begging the question</a>.</p>

<p>I think it's worth asking, not whether it's a good logical proof for the existence of "the unconditioned", but whether it has an effective pedagogical result, e.g. whether it helps the students to understand emancipation from the conditioned. </p>

<p>That reminds me of another fallacy, i.e. <a href="https://en.wikipedia.org/wiki/Zeno%27s_paradoxes#Paradoxes_of_motion" rel="nofollow noreferrer">Zeno's paradoxes of motion</a> which "prove" that you can never arrive somewhere.
Something like that paradox is addressed in the <a href="http://www.accesstoinsight.org/tipitaka/sn/sn51/sn51.015.than.html" rel="nofollow noreferrer">Brahmana Sutta</a> (in that sutta I think that  "park" means "monastery").</p>
