The island of Thera, or modern day Santorini, in Greece. There is a fair amount of information on it online. Basically, it is an island that is the top of a large underwater volcano in the Aegean. At some point it blew, and blew big. At the time, Thera was a thriving nexus point between Ancient Egypt, Minoan Crete and Bronze Age Greece. the blast was big enough that it caused a tsunami that destroyed much of its infrastructure. Thera is more than just a myth though; there have been excavations that reveal the wonderful frescoes and artefacts of this lost civilisation, and proof of its extensive trade links. It is generally though to be the inspiration for the Atlantis myth (ie, it inspired Plato, who made it up). It still exists, and has a modern day settlement. It's a popular destination for tourists, and also for events like weddings. It's a picture postcard kind of place - blue sky, blue ocean, white stucco villas... But with amazing history and archaeology underneath.

There are also the myths of places that maybe kinda exist, but there's not a lot of info. Thule is one example. It's mentioned by Ancient Greek sources as an island in the North Sea, far away and pretty miserable in terms of weather. As a myth, its intriguing, however the reality is likely more mundane; sailors who landed on a particularly foggy bit of Scotland or Scandinavia, or maybe even got as far out as the Faroe Islands or perhaps, at a stretch, Iceland. Thule has been reinterpreted many times since then, mostly becuase the original work that describes it is lost and there are only second hand sources.

There is also the city of Leptis Magna, in modern day Libya. It was a Roman city, inhabited until about 1000AD and then abandoned. It was convered in sand until the early 20th century, when it was rediscovered (on purpose) by an Italian team. Again, go look it up online. It has extensive range of buildings and infrasctructure surviving, including things like mosaics and the port. In the same vein, the discover of Pompeii is a lost city, redicsovered (by accident) in the 16-17th centuries.

Underwater, there is the Greek city of Pavlopetri, discovered in the 1960's by archaeologists. It was occupied from the Bronze Age, through the Minoan and Mycenaean Ages, before it slid under the waves. Much of the information, however, is yet to be published.