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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, in Kentucky, a horizontal cave, is the longest cave system in the world, stretching for at least 390 miles (627 kilometers) beneath the surface of the Earth. However, some caves turn this tidy formula on its head and form from the bottom up. Water trapped in aquifers deep inside the Earth sometimes comes into contact with sulfide-loaded rocks, like pyrite, Orndorff said. This creates sulfuric acid, which, with enough hydraulic pressure, can push up through the rock to carve out a cave.