The Paris Catacombs contain the remains of over six million people. The Catacombs are a network of underground tunnels that stretch for approximately 300 kilometers (186 miles) beneath the city of Paris. The tunnels are lined with stacks of bones and skulls arranged in macabre patterns. The Catacombs were created by moving the remains from overcrowded cemeteries to the tunnels and the process took several decades to complete. The Catacombs were officially open to the public in 1867, and today they receive around 500,000 visitors annually. The Catacombs are open every day except on Mondays, and the visits are guided and last around 45 minutes. The temperature in the Catacombs remains constant at 14 °C (57 °F) and the humidity is around 80%.