Catacombs are underground burial sites that were used primarily in ancient times. They consist of a network of tunnels and chambers where the dead were interred, often in niches or small graves. The most famous catacombs are located in Rome, where early Christians buried their deceased in secret to avoid persecution. These burial sites served not only as places for the dead but also as locations for religious ceremonies and gatherings. Catacombs can be found in various parts of the world, including Paris, where the Paris Catacombs hold the remains of millions of people, showcasing the historical significance of these unique structures.