Curved cement walls, rusted metal grates, pipes, mesh debris collectors, standing water, canals, raised walkway, manhole access points, ladders, graffiti, mold, mildew, oily, stagnant water, waterlogged garbage, pipes, access chambers, tunnels that branch off the main line, shadows, algae, dripping water, rats, spiders, cockroaches, beetles, centipedes, drug paraphernalia, weak light from grates on the street, aqueducts, slime, dead animals/rodents floating in the water, air vents, tunnels, passages, overflow lines high up on the walls to carry runoff, brick walls, branches, sodden plastic bags, leaves
Dripping water, splashes, squeaking rats, echos, footsteps, traffic from above, street sounds, subway running nearby, man made waterfalls to create drainage flow, gurgling water in pipes, roaring water when high capacity (during/after storms), running water, debris scraping along the edges of pipes and catching on grates
stagnant water, sewage (if used partially for sewage), rot, death, pollutants (motor oil, grease and other 'roadside' lubricants that wash in with the street water), the tang of wet stone
That water isn't for drinking, people! Don't lick the walls, either. Might taste sweat on lip
Water seeping into boots, cold water soaking clothing, rats running over boots, sliding hand along a slimy wall, the cold iron rungs of a ladder to the street, bumping unseen debris in the murky water, feeling things brush against your legs, holding a hand to the nose to keep the worst odors out, arms windmilling in an attempt to keep balance on a dry ledge, nudging a dead rat off a walkway into the slimy water so you can step there, swinging a flashlight side to side or up and down, searching for an exit, the side of the sewer scraping along back as you ease down a passage, jerking away to avoid something nasty floating in the water, rubbing swat off your face with the back of the hand because the front you've used to touch the walls and they are filthy, drips of water falling on the head and sliding down your neck
--The words you choose can convey atmosphere and mood.
The beam of my flashlight played off the filth-encrusted walls, the tide line showing where the water had gone down a good four feet. Cold water pressed against my waterproof gear as I entered the offshoot tunnel and an unholy stench slammed into me. I pressed a sleeve to my nose--the blockage I needed to find must be close. After a week's worth of rains causing flooding in half a dozen neighborhoods, there were sure to be a few deceased pets caught up in the mesh debris catch.
Marty balanced on the ledge just above the trough of sludge at the bottom of the tunnel, his heart almost beating out of his chest. He waited in the darkness, straining to hear the race of footsteps above on the street, signalling that Ben and his crew had moved on and it was safe to go back up. His shoes, so precariously balanced on the five-inch ledge, grew heavier and his mind screamed to not look down. He looked anyway and found a well-fed sewer rat sitting on his shoe, cleaning its rubbery tale as if it had every right to sit there and survey its kingdom.
--Similes and metaphors create strong imagery when used sparingly.
Example 1: (Simile)
The storm pipes curved and twisted like the intestines of a giant mythical creature in desperate need of a thirty day, fix-you-right-up detox cleanse.
Example 2: (Metaphor)
Slimy lumps bobbed in the murky brown water, a rancid stew of litter, junkie needles and dead rats.