4. Halemaumau Crater, Hawaii
Halemaumau Crater is a pit crater located within the much larger summit caldera of Kilauea in Hawaii Volcanoes National Park. The roughly circular crater floor is 770 meters (2,530 ft) x 900 m (2,950 ft) and is 83 m (270 ft) below the floor of Kilauea caldera. The gases released at the crater make an immediate impression on one’s sense of smell and taste. The high amounts of sulfur dioxide (SO2) are hazardous to young children, pregnant women, and the elderly and these people should avoid the area.