Iceland is a young volcanic island, so its underground is lava: Grjotagja is a small fissure cave near Myvatn holding a startlingly blue geothermal pool (too hot to bathe in after the 1970s eruptions, and on PRIVATE land, look, do not swim), and Vidgelmir and Raufarholshellir are enormous walkable lava tubes with ice formations in winter. The whole island is a lesson in how fast the Earth can build ground. PUBLIC tours for the tubes, Iceland.
Iceland
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