Rummu Quarry is a flooded limestone pit an hour west of Tallinn, in Estonia, and the water in it was never meant to be there. Prisoners from the Soviet labour camp beside it cut stone here from the 1930s, with pumps running against the groundwater the whole time. When the camp's work ended and the pumps were switched off, the water came up and took the workplace: garages, sheds, fence lines, machinery stopped mid-job. It rose slowly enough that it never knocked the trees over. A stand of them is still upright on the lake bed, dead branches furred with algae that glows green when the sun angles through it. The prison itself stood on higher ground and stayed dry. Inmates went on living beside the flooded pit for another twenty years, looking out at a turquoise hole where the hard labour used to be, and the gates shut for good at the end of 2012.
The shore under your feet is crushed white limestone rather than sand, and it throws the light straight back, so from a board the lake turns transparent beneath you. Roofs, walls and the pale line of an old quarry road slide past a few metres down. Guided divers drop onto the same buildings and carry on into the drowned wood; most of it lies five to seven metres down, shallow enough to give you a long, unhurried look. Above the water rises a hill of waste rock. The path up is loose chip that slides away under your boots, and near the crest the ground turns blinding white, with the whole drowned quarry laid out below like a map. The prison blocks that stayed dry have been painted end to end by street artists and now hold concerts. What the prisoners were cutting is called Vasalemma marble, and it is not marble at all. It is limestone built from the crushed stems of sea lilies that lived here 450 million years ago: pale, hard, good enough to face the presidential office and the parliament, and under Soviet management mostly crushed into road gravel.