The Hydropolis Underwater Hotel and Resort is a planned hotel which will be the world’s first underwater luxury resort. It’s situated 66 feet below the surface of the Persian Gulf, just off Jumeira Beach in Dubai. Reinforced by concrete and steel, its Plexiglas walls and bubble-shaped dome ceilings will enable guests to see fish and other sea creatures. It is basically divided into three sections: the land station, where guests will be welcomed; the connecting tunnel, which will transport people by train to the main area of the hotel; and the 220 suites within the submarine leisure complex. It will cover an area of 260 hectares, about the size of London’s Hyde Park, and will cost an estimated £300 million. It is self-acclaimed to be a 10-star hotel.

From http://en.wikipedia.org/wiki/Hydropolis