Sewers
Source GM Core pg. 95
Sewers are generally 10 feet or more below street level and are equipped with ladders or other means to ascend anddescend. Raised paths along the walls allow sewer workers access, while channels in the center carry the waste itself. Less sophisticated sewers, or sections those workers don't usually access, might require wading through disease-ridden waste. Sewers can be accessed through sewer grates, which usually require 2 or more Interact actions to open.
Sewer Gas
Source GM Core pg. 95
Sewer gas often contains pockets of highly flammable gas. A pocket of sewer gas exposed to a flame explodes, dealing moderate environmental fire damage to creatures in the area.