Caves within Hawai'i Volcanoes National Park are predominately lava tubes, although there are other types of subterranean voids including sea caves, earth cracks, pit craters, volcanic vents, rift magma chambers, caves caused by inflation of lava sheets, and tree casts. Associated with many of these features are sensitive resources that include invertebrates, cultural remains, paleontological deposits, and unusual geologic formations. Nahuku (Thurston Lava Tube) is open every day to the public. Pua Po'o, another lava tube cave, is open to the public only through pre-arranged Ranger led tours every Wednesday. Please call the Visitors Center for reservations. Due to the sensitive and fragile nature of cave resources all other caves within the Park are closed to the general public as well as Park staff not involved in the direct management of the caves. Approved research permits are required for those interested in doing research in caves as well as for interpretive or educational purposes.