Huge Lava Caves Lava Tubes like this were formed by lava flowing underground and hardening along the edges. On the sides of the cave you can see "high water" marks for some of the flows that passed through. When the lava flow ceased, the molten lava inside the tube kept going, emptying out of the solidified skin and thus leaving tunnels. When a Lava Tube collapses it forms sink holes of various sizes. Central Highlands of Santa Cruz Island.
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