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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When Atlantis was sinking, the residents knew about it only slightly in advance. Being sensible people, they rushed into their basements and shut the doors. One atlantis had sunk, they dug between the basements to form a network of underwater/underground rooms. They somehow figured out how to get air down there (and how to build airlocks), and decided there was no reason for them to move. Over the past thousand years, the sea gradually destroyed all of the buildings.

If you know what to look for the doors aren't hard to find, but the lack of above-sea-floor structures make it almost impossible to locate on sonar.
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.