The limestone quarry at Rummu operated for nearly five decades under Soviet administration, supplying construction aggregate to Tallinn and surrounding districts while prisoners from the adjacent Murru Correctional Facility provided forced labor. When Estonia regained independence in 1991, the quarry closed abruptly. Groundwater reclaimed the excavation within months, submerging half-finished prison barracks, guard towers, and industrial machinery in water that now reaches depths of 14 meters. The resulting lagoon spans roughly six hectares, its turquoise hue derived from suspended calcium carbonate particles that refract sunlight in wavelengths between 480 and 500 nanometers. Rummu quarry tickets grant access to one of the Baltic region's most disorienting landscapes — a collision of Cold War infrastructure and accidental natural beauty. The site draws three visitor cohorts: recreational swimmers who wade among half-submerged concrete walls during summer months, technical divers who explore drowned corridors and rusted machinery below the thermocline, and photographers documenting the juxtaposition of decaying Soviet architecture against reed-fringed shorelines. Water temperature stabilizes at 18°C by mid-June, cold enough to discourage casual tourism but tolerable for wetsuit-clad exploration. The quarry's visual drama stems from its industrial abandonment rather than deliberate design. Concrete pillars rise at irregular intervals from the waterline, their reinforcement bars exposed and oxidized. A partially collapsed administrative building tilts at 23 degrees on the eastern shore, its window frames empty, interior walls colonized by birch saplings. The prison complex behind the quarry — Murru Vangla — remains an active correctional facility, cordoned by chain-link fencing topped with razor wire. Visitors photograph the facility's watchtowers from the quarry perimeter but cannot enter the prison grounds. Access via Rummu quarry tours from Tallinn requires 45 minutes by private vehicle along the E265 highway, passing through Vasalemma and Laulasmaa. Public transport options remain limited; most independent visitors drive or join organized excursions that pair the quarry with nearby Pakri Cliffs or Keila Waterfall. The site operates without traditional visitor infrastructure — no marked pathways, no interpretive signage, no lifeguards. This absence of curation preserves the quarry's post-industrial authenticity while demanding that visitors exercise caution on unstable shoreline terrain.