Hibernaculums are locations where animals hibernate for the winter. In the UE realm, hibernaculums are sometimes Abandoned mines or manmade caves.
A common resident of hibernaculums that explorers may encounter is the bat. When abandoning a mine or cave, special gates can be installed to allow bats free access while keeping out humans. This is often done in cooperation with the local Department of Natural Resources to preserve important habitat. During the winter, care should be taken to avoid disturbing bats, as the energy used in waking may deplete their fat stores too much to survive into the spring.
Ammonia: In locations with high bat populations, bat urine can cause very high concentrations of ammonia, which can cause lung irritation. A Respirator may be necessary.
Histoplasmosis: Disease caused by the fungus Histoplasma capsulatum, found in the droppings of bats and birds. Again, a Respirator with a HEPA filter may be necessary for safety. Symptoms can include "a general ill feeling, fever, chest pains, and a dry or nonproductive cough" and can "start within 3 to 17 days after exposure." See http://www.cdc.gov/ncidod/dbmd/diseaseinfo/histoplasmosis_g.htm for further details. (Also see Histoplasmosis)
Rabies: Any time you're near wild animals (mammals specifically), there is the risk that they could be carrying rabies. Avoid touching animals for this reason.