Gunung Sewu, on the Indonesian island of Java, takes its name — which means “thousands of mountains” — from the sweeping landscape of conical hills. The area, which is a UNESCO geopark, stretches 120 kilometers east to west from the hills to the coast. But its real treasure lies deep underground, in a mysterious world of rivers and caverns, adorned with crystals, stalactites and stalagmites and inhabited by unusual creatures. Sculpted by water over millions of years, this subterranean system is a magnet for adventure seekers and a key reservoir for local communities.

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