Lummelundagrottan

The Lummelunda Cave sits in a nature reserve north of Visby on Gotland, and the explored part runs almost 4.5 km, which makes it one of the longest caves in Sweden. Its Swedish name is Lummelundagrottan; it has also been called Rovarkulan, the Robbers' Den.

It was almost certainly formed before the last ice age, but the water that made it is still running through. The cave is fed by drainage from the Martebo mire about 11 km east, and most of that water runs as a stream, the Lummelundsan, through the northern part of the system before emerging from the limestone cliffs known as the Klint, a few hundred metres from the Baltic coast.

That stream used to be far larger. The Martebo mire was once the biggest on Gotland, and draining it at the end of the nineteenth century cut the flow substantially. What remains is carried through an underground canal 1,150 metres long, dropping 17 metres from the mire to Lummelunda, at around 400 litres a second.

Humans put that water to work for centuries. Between the fifteenth and nineteenth centuries there were mills and an ironworks along the stream where it left the cave.

The entrance has been known for a very long time, but the cave proper was not. The main part was found by three teenage boys in the 1940s and 50s, which is a recurring pattern in cave discovery: the entrance is obvious and nobody squeezes past it for generations.

Around 100,000 people a year visit, making it one of Gotland's major attractions. It is cold and wet inside, and the season is limited.