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.
If the Atlanteans wanted to be invisible to radar they could use materials similar to the rubber like tiles used on submarines. If they needed a particularly primitive version of this then large amounts of organic matter, especially seaweed of the sort with the flotation air sacs, would be useful in deflecting and absorbing sonar if strung out above their city.