File:Historic Entrance (Mammoth Cave, Kentucky, USA) 42 (47396257681).jpg

Western Kentucky’s Mammoth Cave is the longest known cave on Earth, with over 412 miles of passages currently explored and mapped. If the 2nd & 3rd longest caves in the world were put together (= Mexico’s White Cave-Two Eyes System & South Dakota’s Jewel Cave), Mammoth Cave would still beat them out by about 50 miles. In a good year, 5 to 6 miles worth of new Mammoth Cave passages are found and mapped. Cave mapping has shown that Mammoth Cave passages are about 300 feet away from caves at Fisher Ridge, where ~125 miles worth of passages are known. Mammoth Cave’s total known length will jump to >500 miles if a connection is made (both sides have “agreed” to not look for a connection). It’s been estimated that, in total, Mammoth Cave may have between 600 to 1000 miles of passages. Casual visitors are only allowed access to about 12 miles worth of Mammoth Cave’s passages.

The name “Mammoth Cave” refers to the immense size of many of the rooms and passages. The name does not refer to its world-record length, nor to the early discovery of mammoth fossils here (actually, fossil proboscidean skeletal remains have been reported from this locality).

The cave is developed in Middle & Upper Mississippian limestones. Partial dissolution of these limestones appears to have started during the late Cenozoic, since at least the Pliocene (the last ~5 million years). Limestone dissolution results from the presence of slightly acidic groundwater. Groundwater originates as rain, snowmelt, or runoff. It percolates through joints and bedding planes of the limestones and slowly dissolves them.

Why is the groundwater slightly acidic? Rainwater is slightly acidic due to the presence of carbonic acid (H2CO3), the result of water-atmosphere interactions (H2O + CO2 --> H2CO3). Once in the ground, water becomes more acidic by picking up complex organic acids from soils and additional carbonic acid. Air occupying spaces in soil is enriched in carbon dioxide gas, so groundwater will end up having more carbonic acid than it did as rain.

Groundwater dissolution has produced a vast network of passages in Mammoth Cave that vary in height, width, and cross-section shape. Passage types include tubular passages, canyon passages, giant canyon passages, keyhole passages, and domepits (vertical shafts).

The elevation of the water table in the Mammoth Cave area has been dropping over the past 5 million years. Currently, the water table in the cave is almost at the same level as the Green River, the main drainage in the area. So, the lowest passages of Mammoth Cave are partially to fully flooded. All the passages above that are dry to mostly dry. The upper dry levels indicate where the water table used to be. Depending on how one counts “levels”, Mammoth Cave has 6 or 7 levels (named Levels A to F).

Seen here is Mammoth Cave's Historic Entrance (looking down from the western rim of the entrance dale). Many of the currently-offered cave tours enter here. The rocks at the mouth of the cave are part of the Beaver Bend Member of the Girkin Limestone (lower Chesterian Series, lower Upper Mississippian). Reported lithologies in the Beaver Bend include lime mudstones (micrites; micritic limestones), some fossiliferous to oolitic wackestones, and lime mudstones having patches of coarsely-crystalline calcite spar (dismicrite). The rocks in the upper part of the photo are quartzose sandstones of the Big Clifty Sandstone (Upper Mississippian).