World’s largest indoor pool

The Seagaia Ocean Dome is the world's largest indoor water park , and is located in Miyazaki, Japan.  The Ocean Dome is measured 300 meters in length and 100 meters in width with a distance of 500 meters from the sea. It opened in 1993, and visitor numbers peaked in 1995 at 1.25 million a year.

Some of the distinct features of the ocean dome include a fake volcano, artificial sand, artificial waves, and the world's largest retractable roof with four sections which permanently shows a blue sky with clouds. The temperature inside the pavilion is always around 30 degrees the temperature of the water at around 28. 

References:

 http://www.kirainet.com/english/the-biggest-indoor-swimming-pool-in-the-worl/

http://www.okeanosgroup.com/blog/uncategorized/japan-holds-worlds-largest-indoor-pool/

http://paulchong.net/2010/04/24/miyazaki-ocean-dome-largest-indoor-water-park-in-the-world-japan/