The Infested Caves are two connected cave systems sealed behind infected glowing roots. To pass, you must use the Sonic Resonator to destroy glowing infection spots, which clears the parasites and opens the route — tying this biome directly into the Proteavirus / infection storyline.
It includes a Spider Dome sub-area. The combination of cramped cave geometry, multiple predators (Marrowbreach, Nibbler Mango) and Bloom Parasites makes this one of the most hostile early-mid biomes despite its modest depth. Do not enter without the Resonator and a clear exit plan.