Every year an estimated 50,000 visitors explore some 910m (995 yards) of tunnel in these dank catacombs to look at 6 million ghoulishly arranged skull-and-crossbones skeletons. First opened to the public in 1810, this "empire of the dead" is now illuminated with electric lights over its entire length. In the Middle Ages, the catacombs were quarries, but by the end of the 18th century, overcrowded cemeteries were becoming a menace to public health. City officials decided to use the catacombs as a burial ground, and the bones of several million persons were transferred here. In 1830, the prefect of Paris closed the catacombs, considering them obscene and indecent. During World War II, the catacombs were the headquarters of the French Resistance.