At what depth are spider cave biomes located in Terraria?
Solution 1:
Vertically-speaking, they can be found anywhere in the underground and cavern levels (between 0ft and ~3100ft in a large map, if you have a Depth Meter). When you start to see Ash Blocks, you've gone too far down.
And they can be found beneath any biome EXCEPT the Jungle. They're slightly more rare underneath the Snow Biome, so you'll have the best luck in the regular stone-and-dirt underground/cavern.
Source, Gamefaqs.
Note: The Dangersense Potion can be useful in locating Spider Caves, because the Cobwebs inside sparkle in a unique pattern.