Beneath the Malpais lava fields runs one of the longest continuous lava tube systems in North America — over 27 kilometres of underground passages where lava once flowed in molten rivers beneath an already-cooled crust. Several of the tubes are open for self-guided exploration, the most popular being the Big Tubes area, a cluster of partially collapsed caves with skylights letting columns of sunlight stream into the dark.
A free permit from the Northwest New Mexico Visitor Center in Grants is required for any caving in the monument; rangers will walk you through current closures (sections are periodically closed for the protection of hibernating bats) and the rules. You need a sturdy helmet, a head torch with at least two backup light sources, gloves and a willingness to scramble on jagged rock. The road in is rough and best in dry weather only.