The biggest underground lake (Namibia)
I have just found out that the biggest underground lake (almost 2 hectares, 20000 m^2) is located in Namibia: ( source: Namgrows 2010 ) It is about 100 m below the ground level and, as you can guess, it is not that easy to visit it! ( source: Namgrows 2010 ) Did I mention that it got an awesome name too? It's called the Dragon's breath !