The rugged El Malpais National Monument consists of almost 200 sq miles of isolated volcanic terrain. There are numerous trails here, accessible via Hwy 53 or the even prettier Hwy 117, though expect very strenuous, exposed hikes (bring a compass). Pick up required permits at the El Malpais & Northwest New Mexico Visitor Center off the I-40 in Grants to explore the lava tubes – some of which are ice caves – or learn about possible hikes.
Five major lava flows have been identified; the most recent one is estimated to be between 2000 to 3000 years old. Paleoindians may have witnessed the final eruptions since local Indian legends refer to ‘rivers of fire.’
You can camp in the backcountry for free, though you'll need a permit. Joe Skeen Campground, 11 miles south of I-40 in in the BLM-managed section of the park, is also a possibility, but bring your own water.
This is an isolated area; it's best not to venture out alone.
The National Monument Information Center on Hwy 53 has closed, but may reopen in the future depending on funding.