Seagaia Ocean Dome, Japan!

Japan has world's largest indoor beach, Ocean Dome about 1,200 miles south of Tokyo in Miyazaki, on Japan’s southern most Kyushu Island.300 meters in length and 100 meters in width, with a height of 38 meters, it can accommodate 10,000 people at a time. 

The beach sits inside a vast dome that measures 300 x 100 meters (984 feet x 328 feet). The heated ocean sends 13,500 tons of salt-free water sweeping across 600 tons of polished marble chips that comprise a long shoreline.

Designed by the Mitsubishi Heavy Industrial Group and opened in July 1993 at a cost of 200 billion yen ($2 billion), the pool has yet to turn over profits, despite catering to daily throngs of tourists.

The Ocean Dome sported a fake flame-spitting volcano, artificial sand and the world's largest retractable roof, which provided a permanently blue sky even on a rainy day. The air temperature was always held at around 30 degrees Celsius (86 degrees Fahrenheit) and the water at around 28 (82 degrees Fahrenheit).

The Ocean Dome Club Alliance Hotels, a group of Miyazaki City hotels, cooperates to provide quality accommodation for Ocean Dome guests. The Ocean Dome is listed in the Guinness Book of World Records as the world’s largest indoor water park, measuring 300 meters in length, 100 meters in width. And the temperature is 30 degrees Celsius year-round. There are quite a few differences between the Ocean Dome and the ocean, among them the heated indoor pool, the kids’ pool, the floating pool, and the three fast and thrilling water slides. Everyone feels like a child when playing in the water.