The World's Longest Lava Tube

The Kazumura Cave system on the Big Island of Hawaii is the longest and deepest lava tube in the world at 40.7 miles long and 3,614 feet deep. Lava tubes form when the outer crust of a flowing lava stream cools and hardens while molten lava continues rushing through the interior. When the eruption stops, the lava drains out, leaving behind a perfectly hollow tunnel. The tubes maintain a constant cool temperature year-round and support unique ecosystems of cave-adapted insects found nowhere else on earth.

Source: Wikipedia