Piusa Caves were created in 1922 as a quarry of quartz sand that was used to make glass. It was quarried underground. Besides the old mining caves there are also three man-made caves in the south from the quarry. The purpose of the caves were geological observations. Piusa caves have evolved into a popular site for hibernating bats. It is the largest known hibernation colony in Eastern Europe.

Source: VisitEstonia.com