For the unfamiliar, there is a subculture of urban exploration surrounding "draining", the act of exploring storm drains and (sometimes) sewers. It can be a fun adventure at times as many drains have large "features" such as massive underground rooms, historic brickwork, cave formations (e.g. stalactites), water storage tanks or waterfalls. Most of the time, they don't even smell that bad and you can get some cool photos. You also often will find oddly-shaped mushrooms growing out of the walls of the tunnels. This is most likely just some group of explorers who were not so careful to observe rule #2: don't be seen entering/exiting the drains. Rule #1 is "when it rains, no drains."