A Hole In The Ground: The Worlds Largest Caves

Caves are judged by a number of different factors-- by length, by depth, by volume, by entrance size, and by chamber size.

The longest cave system in the world continues to be Mammoth Cave in Kentucky (which I have visited). It measures 400 miles in length. It is well worth the trip if you haven't been there: http://www.nps.gov/maca/index.htm Several other caves in the top 10 list are in South Dakota, New Mexico and Mexico.
The biggest cave in the world (with a passage higher, wider and longer than any other cave) is Son Dong Cave in Vietnam. Discovered in 1991, Sơn Đoòng (translates to Mountain River Cave) overtook Deer Cave in Malaysia to take the title of the world's largest cave. A New York City block could fit inside the cave. It also contains dolites. These are formed when the cave roof collapses and vegetation begins to grow inside the cave. Some trees inside Son Doong are 100 feet tall. See National Geographic article for more details: http://news.nationalgeographic.co.in/news/2009/07/090724-biggest-cave-vietnam/
The world's deepest cave (sometimes referred to as supercaves) is Krubera Cave in the country of Georgia. At 7208 feet deep it takes the record from several caves in Georgia. The top 10 are in Georgia (4), Austria (1), France (2), Spain (2) and Mexico (1).

Good cave resources: