Before you go into a sewer, you have to follow a very detailed checklist. You need to gear up in several layers of protective clothing, including hip waders and a disposable jumpsuit. You need multiple pairs of latex gloves, in case the top layer gets dirty, which it inevitably will in a tunnel where every surface is literally covered in human excrement. Then, after donning a hard hat, you need to be lowered down about 20 feet into a manhole. The moment you touch down, the smell is instantly overwhelming.

There are a few reasons to visit London’s sewers, or at the very least, think about them. The British capital has one of the oldest networks in the world—1,200 miles of