Deep in the bowels of the London Bridge tube station is a maze of cavernous rooms which have been turned into a late-night bar/nightclub/hangout space. You won't need hallucinogenics if you go to The Shunt Vaults...the atmosphere is trippy enough to make even the most sober attendee wonder if someone's slipped something into their drink.-
Though it stays open late, Shunt stops letting people in at midnight (when the Tube Station closes). When the clock strikes 12:01, the big doors close and even if you wish to leave immediately, you'll have to find your way through room after dark room til you reach the exit which spits you out under the bridge.
After coat check, the first stop at Shunt should definitely be the bar (there are multiple throughout the vaults, all with different liquors), because you'll definitely need a drink as you wander though the cavernous spaces. The venue is often used for art purposes, so it's artists and not kitschy club designers who are in charge of the decor.
Each room is different; when I was there, one was set up to look like the interior of a commercial plane (seats were complete with trays and seatbelts), one was set up with a floor covered in sawdust and glitter, one was full of laz-y-boys. And then there was The Fairy Room.
The Fairy Room consisted of a tea party, where actors dressed as fairies sat stone-faced around a table, not talking. There were empty chairs peppered throughout the actors, so intrepid (read: drunk) guests could take their seat among the fairies. (I sat. I stared at them. They stared at me. We contemplated each other. I headed to the shot bar.)
Later in the evening, the band of wandering fairies traveled from room to room, rolling around orgiastically in the sawdust, prancing about, dancing. It was, needless to say, surreal.
In other rooms, light sensors were set up to produce beats when you walked across them, so the throbbing music perfectly matched the motions of the crowd. Wine bars, shot bars, and beer bars, all tended by friendly bartenders who are happy to keep your glass full once you buy an initial drink.
Room after room of bands, installation art, dancing and alcohol had foreigners and Londoners alike lifting their glasses to the geniuses who thought a Medieval (seeming?) bridge would be an ideal place for a nightclub.