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Magma in Earth's Mantle Forms Deeper Than Once Thought. Study simulating pressures in mantle beneath the ocean floor shows that rocks can melt at depths up to 250 kilometers. When magma reaches Earth's surface and erupts from a volcano, it becomes lava.
Mammoth Cave near Cave City, Kentucky, is the world's longest cave. More than 367 miles (591 km) of passages have been mapped. To get an idea of how long Mammoth Cave is, you should consider that the second longest cave in the world, Optimisticeskay Cave, in the Ukrainskaya region of the C.I.S. (former Soviet Union), is only 143 miles (230 km) long. Mammoth Cave is three times longer than its nearest competitor. Not only is the cave the longest in the world, but cave mappers believe the cave system will eventually prove to be 560 miles (900 km) long! Mammoth Cave was formed in Mississippian-age limestone rock. The cave is 379 feet (118 m) deep, and contains at least 5 levels of passages.