user86725: I'm wondering if all the hashes of a previously unlocked difficulty level of a cryptocurrency, such as 

0000000000000000xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx BTC

, have been completely mined, where x is a placeholder for a constituting HEX digit, such that all the possible combinations of digits in that difficulty level have been mined.

user7272: No, and this will not happen in the foreseeable future.
Any given hash calculation will result in any one of 2^256 possible numbers. That means, the odds of finding any one exact number is 1/2^256.
Even at today's hashrate of ~47000000 TH/s, it would take you 7.812×10^49 years to find a preimage for every one of those 2^256 numbers, assuming each hash you find is unique (no collisions).

86725: <p>I'm wondering if all the hashes of a previously unlocked difficulty level of a cryptocurrency, such as </p>

<blockquote>
  <p>0000000000000000xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx BTC</p>
</blockquote>

<p>, have been completely mined, where x is a placeholder for a constituting HEX digit, such that all the possible combinations of digits in that difficulty level have been mined.</p>
 7272: <p>No, and this will not happen in the foreseeable future.</p>

<p>Any given hash calculation will result in any one of <code>2^256</code> possible numbers. That means, the odds of finding any one exact number is <code>1/2^256</code>.</p>

<p>Even at today's hashrate of ~47000000 TH/s, it would take you <code>7.812×10^49 years</code> to find a preimage for every one of those <code>2^256</code> numbers, assuming each hash you find is unique (no collisions).</p>
