what you are looking at
Spider Nests are Cavern layer pockets with the darkest background walls in the game and enough web packed into them to fill every gap. The web is the point. In front of those walls Cobweb grows back on its own, which makes a nest the only place in the world that regenerates Silk supply. Sizes run from around 500 tiles up to just under 3500, so one nest is a side room and the next takes twenty minutes to cross.
There is a catch on harvesting. Web sitting in front of spider walls only yields an item one time in four, while web anywhere else in the world drops every single time. Plan on breaking four times as much as you collect, and do not expect a nest to be a fast source of silk so much as an endless one.
the walls are the biome
Every property of a nest comes from that one wall tile. Spiders spawn because of it, web regrows because of it, and taking a hammer to it removes all of that at once. That is the fix when a nest has generated somewhere inconvenient, and it is also the reason not to casually clear walls in a nest you intend to farm.
the reason to come
Spider Fangs drop from the Wall Creepers and Black Recluses that live here, and they buy the summoner set that carries the class from late pre-Hardmode through the mechanical bosses. The Spider Mask, Breastplate and Greaves add minion capacity and minion damage on top of ordinary defense, and the Spider Staff and Queen Spider Staff come out of the same pile of fangs. One farming trip kits out an entire class.
The Webbed Stylist is here too, wrapped up and hanging from the ceiling. Cut her down and the Stylist moves into town. Web Covered Chests turn up rarely and are worth the detour when you spot one.
getting through one alive
- Bring far more light than feels necessary. The walls swallow it, and a spider you cannot see is a spider already on you.
- Spiders climb walls and ceilings and drop straight down. Look up before you commit to a corridor.
- Web slows you to a crawl while things that ignore web close in. Clear a lane before you choose to fight in one.
- Slope a three block high tunnel with a hammer and spiders cannot pass through it, though you walk it normally. A two block wide vertical shaft does the same job.
Hellevators are the other common collision. Punch a shaft down through a nest and the regrowing web catches you mid fall every time you use it afterward. Line that stretch with rope or torches so there is nowhere for web to form, or hammer the wall out where the shaft passes through.
finding one
A Dangersense Potion flags cobwebs as a hazard, which quietly makes it a nest detector. Drink one while exploring the Cavern layer and a nest lights up through solid rock well before you reach it. In worlds running The Constant, spider walls also creep up to the surface, so you may not have to look at all.
Once you have one cleared of tiles but not of walls, the constant respawns make it a decent money farm, since everything that spawns there drops something and nothing else competes for the spawn slots.