Just beneath the ground in the Piusa Sand Caves, is a massive group of hibernating bats. Having made their home in the man-made passages since the 1950s, the size of this protected bat house has swelled to become the largest bat colony in all of the Baltic States.

Beginning in 1922, miners began to hollow out the area that is now the Piusa Caves. Excavating for glass-sand, miners carefully hollowed out the sandstone into long corridors, open galleries and surprisingly elegant vaulted ceilings. Starkly different from other mines that are blasted apart or simply hacked to death, the Piusa Caves almost seem like a planned underground development.

Whether the bats thought so or not, they moved in, and have been protected in the caves since 1958. By 1970, mining had tapered off completely, and the caves are now only known for their bats. Counted at a remarkable 3000 bats in 1999, five different species of bats have flourished in the underground world, causing many scientists to come to Estonia to study them.

Although the caves can be explored, visitors are warned to stay on marked paths only to avoid injury in the delicate caves, and to not stir up the flying mammals.