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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.
A cave or cavern is a hollow place in the ground, especially a natural underground space large enough for a human to enter. Caves form naturally by the weathering of rock and often extend deep underground.