Lake Vostok in Antarctica
It's difficult to answer because it depends whether you count all the aquifer systems, subterranean pockets discovered by sonar, just those inside caverns within cave systems, etc. The latter is in Namibia but there are likely larger ones undiscovered because the Earth's crust isn't exactly accessible overall.
The largest fresh water supply in the world is under the Midwest U.S. (NE, IA, MN, SD).
The country with the largest supply of fresh water in the world is said to be Brazil, followed by Russia, Canada, the United States, Indonesia, and China.
Sarawak Chamber is largest underground chamber in the world by area. It is in Borneo.
"The Antarctic Icecap is the largest supply of fresh water, representing nearly 2% of the world's total of fresh and salt water." Source: http://www.infoplease.com/ipa/A0004674.html
in toronto ---- Montreal's underground city is the largest underground complex in the world.
The London Underground is the largest in Europe and the oldest in the world.
The level at which all spaces inside underground rock are filled with water is called the water table.
carlsbad caverns
Antarctica's ice sheet is the largest supply of fresh water in the world.
carlsbad caverns
http://winecountry.blogspot.com/2006/04/largest-underground-wine-cellar-in.html
Seawater, freshwater, and underground water