Imagine discovering a lake whose waves gently lap at its shores, leaving high water marks and ripples in the nearby sand. Now imagine the very same scene, located more than 650 feet below the surface of the Gulf of Mexico.

Scientists plumbing the ocean’s depths just off the Yucatan Peninsula stumbled upon just such an unfathomable sight when, in otherwise dark waters, their submersible’s lights were reflected by the bright white, mirrored surface of the undersea lake.

Composed of super-concentrated, denser-than-water brine (not to be confused with the ocean’s run-of-the-mill “briny blue” seas), these “brine pools” are thought to form as the byproduct of salt tectonic shifts dating back to the Jurassic period. Due to lethal levels of hyper-salinity, only bacteria and other microorganisms are able to live within these underwater lakes. Mollusks and crustaceans are indigenous to their shores, though crabs and urchins placed in the pool during scientific testing were, effectively, pickled.

Those who have witnessed the pools first-hand report that the liquid is so dense it feels as if submarines bump into it rather than glide through.