This is a so-called "brine pool" - an area of very concentrated (hence very dense) seawater, at the bottom of the ocean. It also contains methane and hydrogen sulphide, amongst other things, so it's toxic to most forms of life, hence the nickname. This one is at the bottom of the Gulf of Mexico, about a day's sail from New Orleans.
Further reading and viewing, for those interested:
- an informative
article on Seeker.com
- some
pictures on the Nautilus website (the exploration submarine that discovered the pool)
- a YouTube video of the Nautilus crew exploring the pool
