The documentary Dark Secrets of the Black Sea journeys to the said region and explores recently-unearthed archaeological evidence of a technologically advanced civilization that once lay there, now submerged beneath the Black Sea. The Black Sea has been the focus of many archaeological and underwater expeditions due to its unique properties and history.

The Black Sea experienced significant changes in water levels, especially around the end of the last ice age. Ancient coastlines, now submerged, could contain settlements or remnants of human habitation. This offers a fascinating avenue for underwater archaeology.

One of the unique features of the Black Sea is its anoxic (oxygen-free) deep layer. Organic materials, like wood, can be preserved in this environment for millennia. This property provides an opportunity for remarkably well-preserved ancient shipwrecks and other artifacts to be found in the deeper parts of the sea.

Some researchers have postulated that changes in the Black Sea's water levels might have been the basis for various flood myths, including the biblical Noah's Flood. A significant and rapid influx of water from the Mediterranean, potentially flooding previously inhabited areas, could have been remembered and passed down in oral traditions.

While there have been numerous shipwrecks and artifacts discovered in the Black Sea, there hasn't been a discovery of a large, mysterious underwater city. Nonetheless, the history of human habitation around the sea is vast, and there are likely many submerged sites awaiting discovery.

It's crucial to differentiate between speculative theories and verified archaeological findings. While the Black Sea is full of potential, claims of vast underwater cities or connections to legendary places like Atlantis are, as of now, in the realm of myth and speculation.