Former quarries often flood, creating swimming holes that are perfect sites for leaping off of cliffs and horsing around with friends. Unfortunately, they can also have some nasty residues left over from their industrial days.
Harpur Hill Quarry, a former limestone quarry in Derbyshire, England, is one such spot. From 1835 to 1952, limestone was extracted from the site and processed to create quicklime, a chemical compound used in steelmaking. In 1938, the quarry was taken over by the Royal Air Force and used as a depot for storing chemical weapons. The depot operated through both World Wars, and eventually shut down in 1960.
Tourists were drawn to Hoffman Quarry—more commonly called the Blue Lagoon—because of its vivid turquoise blue waters. Finely dispersed particles of calcium oxide, a remnant of the quarrying process, left the water with a hue that looks like it came from a Caribbean beach. But caustic quicklime dissolved in the water means that the lake has a highly alkaline pH of 11.3. (For reference, ammonia has a pH of 11.5 and bleach has a pH of 12.6.) If that is not enough, the site has been used as a dumping ground. Signs leading up to the lagoon read: “Warning! Polluted water. Lagoon known to contain: Car Wrecks, Dead Animals, Excrement, Rubbish” and “Warning! Do not enter water, due to high pH levels. This can cause: Skin and eye irritations, Stomach Problems, Fungal infections such as thrush” and “Think! would you swim in ammonia or bleach?”
But the draw of the turquoise water, turned green as limestone rocks leached calcite crystals into the water, was irresistible. Families drove to the lagoon and children regularly swam in it with the simple warning not to dunk their heads or swallow any water.
Locals have been trying to deter swimming in the quarry for years. The lagoon is reportedly too toxic to be drained, but in 2013, the county council added black dye to the water in an effort to deter swimmers. While it seems to be effective, the dyeing process has to be re-done every few years as the water returns to its alluring turquoise hue.