Entry into storm drains, or draining, is another common form of urban exploration. Groups devoted to the task have arisen, such as the Cave Clan in Australia. Draining has a specialized set of guidelines, the foremost of which is "When it rains, no drains!" The dangers of becoming entrapped, washed away, or killed increase dramatically during a heavy rain fall.
A small subset of explorers enter sanitary sewers. Sometimes they are the only connection to caves or other subterranean features. Sewers are among the most dangerous locations to explore owing to extremely high risks of poisoning by buildups of toxic gas (commonly methane and hydrogen sulfide). There have been numerous fatalities around the world where people are overcome by toxic gas from sewers.