### Kazumura Cave
USA (Hawaii) — Formed 500 years ago by a flowing river of superheated magma, this giant, glass-walled lava tube drops 1,102 meters.
Most caves are carved over millions of years by the slow, chemical drip of acidic water. Kazumura Cave was carved by fire. Roughly 500 years ago, a massive effusive volcanic eruption from the Kīlauea volcano sent rivers of glowing, basaltic lava rushing down the mountainside. As the flowing magma hit the cool Hawaiian air, the outside chilled and hardened into a thick rock crust, creating a perfect thermal insulator.
Inside this newly formed rock pipeline, the superheated liquid rock just kept flowing like a subterranean river. Because the crust trapped the heat so incredibly well, the magma only cooled about 4°C during its entire 39-kilometer journey. When the eruption finally stopped, the molten rock simply drained out the bottom, leaving behind a colossal, hollow tube.
Today, Kazumura isn't just a playground for Earth scientists—it's a testing ground for planetary geologists. Satellites have spotted massive "skylight" holes on the Moon and Mars, and scientists suspect they are giant, extraterrestrial lava tubes. Researchers study the inflation and overcrusting processes that formed Kazumura to debate a massive mystery: since Mars has lower gravity than Earth, could alien lava tubes be big enough to house entire human bases without collapsing?