The Catacombs tour takes about 45 minutes and is not for the faint of heart. It covers a little over a mile (2 kilometers) underground with 130 steps down and 83 back to street level. The real exertion may be standing in line waiting to get in, especially in the summer. Only 200 persons are allowed to enter at any one time. Sometimes booking an escorted tour ahead of time lets you jump line. Check before you book. It can get cool down here, so pack a sweater.