Gas-warning sign in Hawai‘i Volcanoes National Park, Hawaii

Warning sign for carbon dioxide gas located on a hiking trail just below Kīlauea Crater rim. The odorless nature of CO2 and its tendency to pool in low-lying places make it an insidious hazard for people wishing to enter summit lava tubes, and the National Park Service closed them to casual entry in 2006. The park also now requires that those applying for a permit to enter lava tubes take extra precautions, including wearing personal CO2 monitors. When the Overlook vent in Halema‘uma‘u Crater became active in 2008, the Park Service closed access to the floor of Kīlauea Crater.