Seismologists have identified a massive body of water the size of the Arctic Ocean beneath eastern Asia deep within the Earth's 'mantle'. The ocean has been nick-named the 'Beijing anomaly' because the seismic signature which indicates the presence of the underground ocean is strongest beneath the Chinese capital city of Beijing.
It was also recently discovered that in addition to the great Amazon River, there is a second river that is draining the Amazon basin. This river, the Hamza, is every bit as large as the Amazon River, only it is 13,000 feet below the surface. The river has been named after Valiya Hamza, the scientist of Indian origin who has studied the Amazon region for more than 40 years.
Equally interesting, though perhaps a bit more fringe, are tales of a vast underground network of massive aquifers and rivers that drain the entire southwestern United States. This underground network is purportedly navigable and empties into the ocean along the coastline. There are rumors that two U.S. military submarines (U.S.S. Thresher & U.S.S. Scorpion) were sent into these underground rivers to chart their course and determine their origin, but the submarines were apparently 'lost' never to be heard of again.