A remnant of an ancient volcanic explosion, Kilbourne Hole Volcanic Crater was designated a National Natural Landmark in 1975. This crater can be found in a desert basin between the Potrillo Mountains and the Rio Grande in southern Dona Ana County. The area is part of the Organ Mountains-Desert Peaks National Monument, managed by the Bureau of Land Management. Kilbourne Hole is roughly elliptical in shape, and is known as a Maar — a pit or depression caused by a volcanic explosion with little material emitted except volcanic gas. The crater is between 24,000 and 100,000 years old, and measures 1.7 miles long by well over a mile across, and is hundreds of feet deep. Kilbourne Hole has many unique minerals remaining from the eruption. The crusts of the Kilbourne Hole volcanic bombs — objects blown from the volcanic vent in a near plastic state — are dull black or brown, but when broken open they often reveal a brilliant, sparkling yellow and green interior of olivine glass granules. Visitors are encouraged to hike the area, however, there are no restrooms or drinking water available. The area is open year-round and there are no fees.