It is still a myth as far as our modern civilization is concerned, Atlantis located somewhere within the devil's triangle and it's inhabitants had struck down many planes and vessels daring to disturb the peace. Without resorting to magic how can Atlantis disappear from the sonar? (Answer with most primitive tech wins.)
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> Atlantis located somewhere within the devil's triangle and it's inhabitants had struck down many planes and vessels daring to disturb the peace. Without resorting to magic how can Atlantis disappear from the sonar?
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You **already** need magic or very advanced technology to justify a city existing **underwater**. However, given that Atlanteans somehow either have or don't need air...

* Once submerged, the city might have just decayed away - it wasn't built of stone but of unbaked clay bricks or organic matter. The Atlanteans now live in smaller structures that sonar can't resolve.
* The submersion was slow enough that those Atlanteans as didn't want to simply relocate, and saw the futility of building taller and taller structures above ground, built instead deeper and stronger abodes below what is now the sea floor.
* They might have their city covered, naturally or artificially, with a sound-reflecting layer that mimics the sea-floor (for insulating purposes, for example: several thermal insulators are also good sound insulators). They might not even be aware of this effect, unless informed by friendly cetaceans (if they were, could we say they did this on *porpoise*?)

Atlanteans still can hit ships with colossal bubbles of methane, that have the same effect on a ship as a torpedo (they make the supporting water underneath disappear, so that the ship's weight cracks the vessel in the middle). Additionally, a delayed fuse can turn the bubble into a fuel-air bomb, capable of taking down a plane if it flies low enough.

The problem, possibly requiring magic, is now *how they know that there are ships and even planes above*, and where very exactly they are and how fast they're going - it takes time for a methane torpedo to hit.
In an old short story I wrote years ago (but never published because my writing was *meh*), I figured that Atlantis was sitting on the top of a volcanic island and during an earthquake, the city that had overdevelopped to the point of weakening the soil, fell into a nearby oceanic trench upside-down. The city remained mainly intact, but the city acting like a plug at the top of a bottleneck, the citizens never could escape to the surface again.

Here is a pic of what I mean (sorry for the language in the image, I was on the version of [this wikipedia](https://en.wikipedia.org/wiki/Abyssal_plain) article when I took it):

[![enter image description here](https://i.stack.imgur.com/By0i9.png)](https://i.stack.imgur.com/By0i9.png)

Thus making the city undetectable by any means without resorting to magic.