I'm still in Puerto Peñasco, Mexico. Our mission today is to visit the El Pinacate Biospheric Reserve, a vast, arid region of mountains, sand dunes, and desert that takes up most of the territory between the northern tip of the Sea of Cortez and the Arizona border. By my calculation, about four Nutmeg States would fit inside. Extraordinary by any standard, El Pinacate (peen-a CAH-tay) would be a household name if it were in the U.S.

In fact, it very nearly was. Remember the 1854 Gadsden Purchase? We bought what is now southern Arizona from Mexico, mainly to build the Southern Pacific Railroad. The original deal was a blockbuster, including all of the Baja Peninsula and large tracts of northern Mexico. It didn't all go through for many reasons, but if it had, El Pinacate would have become an American national treasure.

Skeptical? OK, then, let's go: It's about 30 minutes out of town.

Boulevard Benito Juarez is Puerto Peñasco's Post Road. It begins at the Old Harbor, a district of shrimp boats, fishmongers, restaurants, and souvenir shops, and winds north past the modern City Hall and the Plaza del Cameron. The Plaza pays homage to the shrimp -- that tasty crustacean that has been the town's lifeblood -- with a giant, anatomically correct sculpture.

Juarez then straightens out into a colorful, erratically paved thoroughfare for all manner of vehicles and pedestrians. It's lined with business establishments ranging from tastefully upscale to humble, from familiar to unexpected. Examples: A cafe featuring Viennese strudel; a tequila factory; thatched-roof chicken joints (there is no better grilled chicken anywhere, believe me); makeshift taco and juice stands; a car wash; a U.S.-style supermarket; a baseball field; plumbing, tile, and electrical stores; deserted stores; half-built stores; dentists (many Americans cross the border for dental care); a veterinarian; and a Burger King. This is the busiest street in town, but driving here is relaxed, as Mexicans are polite and patient, even behind the wheel. At red lights, a street vendor may offer you strawberries, oranges, or asparagus.

At the north edge of town, past a brand-new big-box store owned by Wal-Mart, Juarez becomes Route 8. In minutes, beyond a few RV parks and unfinished structures of uncertain function, it's just you, Route 8, and the desert. It's on this arrow-straight stretch of road that you find the El Pinacate Biospheric Reserve.

The austere, contemporary Centro de Visitantes springs up at the end a four-mile drive through a starkly beautiful expanse of desert, a "forest" of cactus. There, you step out of your car into a different world. The visitor center is at the southern edge of thousands of acres of lava flows that have been rolling over this landscape for 4 million years. The El Pinacate mountain range is extensively volcanic, and so distinct that it can be easily seen from space. The risk of death by molten lava is low -- the most recent lava flow was 11,000 years ago, give or take a few centuries. There are two short trails from the visitor center that take you around and over striking formations of lava that support an amazing variety of plant and animal life.

As remarkable as the Centro de Visitantes area is, the big show is further up Route 8 -- the entrance to the internal route through the El Pinacate Reserve. After ponying up 50 pesos ($4) per person, you set out on an unpaved, 40-mile loop road through terrain you would be hard-pressed to find anywhere else on earth.

Inside, you're on your own. There are no snack bars, souvenir shops, or motels. When you stop to look around, it's easy to become unsettled by the utter silence and remoteness of the enormous desert plain. Few environments could be as unfriendly to human life, but there is evidence of human habitation dating back as many as 40,000 years. Maybe these desert people liked the views -- which are sensational, framed by clear blue skies and jagged, forbidding mountains that are positively lunar -- so much so that NASA used this desert to train astronauts. Every view is dominated by El Pinacate itself, 4,000 feet at its peak.

The road then climbs into a section of desert carpeted with powdery, chocolate-brown volcanic dust, a stunning contrast to the desert growth, especially with highlights of white, orange, yellow and magenta wildflowers thanks to winter rains.

But what about the volcanoes? I place in evidence El Elegante Crater, the largest of many in the reserve. You walk up an unassuming path, and suddenly, you're peering slack-jawed into a crater of staggering dimensions: a mile across, and 600 feet down to the bottom. There's no way to prepare for the dizzying scale of it unless you've seen the Grand Canyon. The trail that goes part way around the rim is hypnotic.

As if El Elegante wasn't amazing enough, there are two other equally heartstopping craters to visit along the road, El Tecolote and Cerro Colorado. Four hours after we set out, we emerged on Route 8 in the kind of trance that could only be broken by shrimp tacos and a cold beer.

Ron Blumenfeld is a Fairfield writer and retired pediatrician. His "Moving Forward, Looking Back" appears every other Wednesday. He can be reached at: rblumen2@gmail.com.