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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.
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.