Thurston Lava Tube at Hawaii Volcanoes National Park, Big Island, Hawaii. The step mark on the right wall indicates the depth at which the lava flowed for a period of time.
Inside Thurston Lava Tube, the step mark on the wall shows exactly how deep the lava once ran — an eerie, unforgettable window into what built this island. © Frank Schulenburg, CC BY-SA 3.0, via Wikimedia Commons

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