Most of the city's 19th century houses were built of limestone mined nearby. Abandoned mines were later used and broadened by local smugglers. This created a complicated labyrinth of underground tunnels beneath Odessa, known as "catacombs". They are a now a great attraction for extreme tourists. Such tours, however, are not officially sanctioned and are dangerous because the layout of the catacombs has not been fully mapped and the tunnels themselves are unsafe. These tunnels are a primary reason why subway was never built in Odessa.