What's one memorable place you've been or one unusual thing you've done below ground level — in a cave, tunnel, subway, or basement?

I went inside tombs in Israel. I camped inside caves in Tennessee.

I like the underground shopping area in Montreal. I’m not all that into shopping, but it was a cool experience to walk around down there.

I have, on two separate occasions 5 years apart, been to two different opium dens, both of them located below ground. And, yes, I smoked opium in both of them.

The first one was located in the basement of the old mansion in Laurel Canyon where Fatty Arbuckle (a comic film actor in the 1920s) had his home and where the infamous party took place that ruined his career. It was dark, old, dirty and smelled of opium and charcoal. The walls were black and red lacquered with a few Chinese style illustrations.

The second one was located in some secret tunnels underneath Chinatown in San Francisco. The entrance was through a hidden door in a brick wall in an alley. The tunnels were very clean with a surprising amount of fresh air circulating throughout, apparently due to an underground stream of fresh flowing water which you crossed over every so often on fancy little Chinese style bridges. The actual opium den was also very clean. Everything was lighted with small natural gas lamps recessed into the off-white clay walls. It was extremely quiet and peaceful.

Underground shopping in Toronto. It's extensive: going on for miles.

Catacombs in Rome.

Salt Cathedral in Poland.

When I was 5 and by brother was 7 we dug out an underground fort for ourselves. Thank goodness it didn't collapse on us because weren't supervised.