Rummu Quarry is a flooded limestone quarry in Harju county, an hour west of Tallinn in Estonia, worked by the prisoners of a Soviet labour camp until the pumps stopped in the 1990s and the groundwater rose over the workshops and the trees, and its turquoise water now stands over sunken garages, machinery and a forest still upright on the bed, paddled over on boards and dived with a guide. The quarry supplied limestone from the 1930s, and the prison beside it, Murru, held the men who cut it; when Estonia regained its independence the quarry was abandoned, the pumps were switched off and the water rose in a few years, swallowing the workplace but not the prison, whose buildings stayed dry on the higher ground and closed in 2012. The shore is crushed white limestone rather than sand, and it throws the light straight back, so from a board or a boat the lake turns transparent underneath and the roofs of the drowned buildings show a few metres down, with the walls of the old quarry face rising on the far side.
Guided dives drop onto the service buildings the water caught, garages, sheds and machinery stopped mid-job, and carry on into a stand of drowned trees whose branches are thick with algae, in water that stays cold below a thin warm layer all year. Beside the lake rises a hill of waste rock, a white pyramid of limestone chip that slides underfoot on the way up, blinding near the crest, with the lake, the prison and the flat country of Estonia below it. The prison buildings that stayed dry have been painted end to end by street artists, and the site holds concerts and open-air events among them in summer, with a beach on the limestone shore where the surface reaches twenty degrees in August. The lake is only turquoise when the sun is high, and the low northern sun of winter leaves it grey. Rummu is a prison that drowned its own work, and the water made it a beach.