If the walls of Mønsted Kalkgruber could talk, they could say a lot about the thousands of people who have come through the mines over the centuries, but probably even more about the thousands of bats that currently live there.
The caves are very, very old. When Denmark was becoming an increasingly Christian nation around the 11th century, limestone mining was a profitable industry because the stone was used in cathedrals. From this time up until the 19th century, the miners used practically the same technique of assembly line-style limestone ferrying. Machinery was introduced in the mid-1800s, individual mines merged together, and the tunnels were mined more extensively. Limestone remained profitable until the mid 20th century. The mines closed in 1953.
The caves changed hands many times in the years following the quarry's closing. At one point they were property of Anker Buch, a concert violinist who staged performances in the acoustically accommodating caves, a practice which continues even now. In addition to cultural events in the caves, a museum dedicated to showcasing Denmark's oldest industry also operates within the mines.
Sixty kilometers (about 37 miles ) of underground paths comprise the caves of Mønsted Kalkgruber, though only 2 kilometers of that are electrically lit. The tunnels vary wildly in size: Some are cathedral height, some are low enough that an adult can't walk through upright. These tunnels open up into various cave "rooms", some of which contain entire underground lakes. Visitors to the Mønsted Kalkgruber museum can wander through the caves on their own or take a train ride tour throughout.
The train rides are only available between May and August however, out of respect to the caves' inhabitants: some 18,000 bats. In spring and summer the bats fly all over Jutland, eating insects to their hearts' content. In the fall and winter months the bats retreat to the caves, when they need quiet for hibernation (hence no trains). During the beginning of the cold season, they tuck themselves into crevices in the caves, out of view from the museum visitors. In the latter half of the season though, the bats emerge from hibernation to test the climate. They fly about and hang from the ceilings as they wait for spring. The Mønsted Kalkgruber museum is intent on preserving the bat population, so much so that they feature a bat counter, noting each bat's entry and exit from the caves.