The Spider Caves of Zzyzzx are a location in Sparks Nevada Marshal on Mars .
The caves are on Mars, and are a stronghold for giant spiders.
In Inside Out in Outer Space... Again!, Lycos Bill flees to the caves after shooting Sparks Nevada with the Inversifier, while planning on stealing treasure from the spiders who lived in the caves. Bill turns the caves inside out, thereby releasing all of the spiders to the outside world.
He returns the caves to normal but it is too late. Bill is quickly swarmed and killed by the spiders. Croach the Tracker wades into the spiders to retrive the Inversifier. He manages to return it to The Red Plains Rider before being carried off to the depths of the caves.
Red returns to the caves with Sparks in order to save Croach. Using infra-red goggles in the very dark caves, they are able to find Croach but are quickly captured by spiders themselves. Billy the Bot, looking for treasure nearby, hears their cries for help and kills most of the spiders on his way to find the trio. He eventually releases them from captivity.
Some spiders survived the massacre, such as Kevin, who later went on to fail to extract revenge on Sparks for killing his parents that day.