The sounds of artillery pepper the air. Even though they’re far away, the detonations send a shockwave across the desert expanse, putting everyone on edge just the slightest bit. Our cameras stop clicking, we look up from our field books, pause for a nervous moment, and resume our recon work.

You see, we’re among the first people from the outside to ever get an up-close look at the General Motors Desert Proving Ground Yuma, a 2,500-acre expanse leased from the United States military. Everything about the Yuma facility seems clandestine, from the camouflage netting that lines the exterior fence to the suspicious glances we get from its fulltime employees. And then there’s the persistent shelling (Desert Proving Ground is surrounded on three sides by military test facilities). The sound makes you wonder if you’d last very long if you were to somehow make a wrong turn and end up outside the grounds.

All the secrecy keeps prying public eyes away from GM’s most valuable intellectual property (including the subject of our visit, the Chevy Colorado ZR2 ), and the grounds’ location is ideal for vehicle testing: its military status turns it into a gigantic no-fly zone. According to facility employee Douglas Moore, GM’s old desert proving grounds occasionally had to deal with enterprising journalists who’d charter small planes or helicopters to capture spy photos of new vehicles from above. Try that now and you’d be stuck with an F-18 escort to the nearest detention facility within about five minutes.

Turning the barren desert land into a modern proving ground took GM engineers about a year, developed using a global template shared with the company’s test facilities in China and other regions. The design allows GM to repurpose vehicles built for other markets locally; for example, the China-built Buick Envision was tested in that country for U.S. consumption, with GM’s engineers sharing data with their American counterparts. Using similar proving grounds and identical test schedules allows GM to develop vehicles for a worldwide audience faster and with less intercontinental commuting.

View Photo Gallery | 33 Photos Photo 2/33 | The Saguaro Trail snakes past "The Graveyard," a collection of engines and car parts engineers found on the Yuma site when building Desert Proving Ground. That frame in the photo? It's from an early ZR2 mule, accidentally mangled by an anonymous GM exec.

While that numbers-oriented approach can take a vehicle development program pretty far, at some point, engineers need to drive by the seat of their pants and ignore engineering parameters. So when GM started development on the Chevrolet Colorado ZR2, its designers encountered a problem: Yuma’s graded dirt roads and washboards posed zero threat to the ZR2’s prodigious off-road capability ( credit its trick shocks ). In order to really put the off-roader through its paces and ensure it was up to the task of real-world rocks and dirt, they needed a new test site. Enter the Saguaro Trail.

Built more or less exclusively for the Colorado ZR2 , the Saguaro Trail came online in 2015, turning unused property into a 6-mile off-road test loop. The Saguaro Trail is man-made, but its off-road features were inspired by real-life trails and profiles, incorporating three four-wheel launches, one mile of sandy wash, a tabletop jump, several twist ditches, a wide-open Baja-style trail, large swells, and a washboard grade. According to Performance Engineer, Ride and Handling Brad Schreiber, the trail is as repeatable as you can ask for within a natural environment.

That “natural environment” is key, because unlike the rest of the clinically sterile Desert Proving Ground, the Saguaro Trail is heavily affected by rain, wind, and wear and tear from vehicles. As such, raw lap times don’t mean as much here as they do, for example, on the Milford Proving Ground’s paved Vehicle Dynamics Test Area. Rather, GM engineers wind out the ZR2, evaluating how it feels over the Saguaro Trail’s unique test surfaces as they change over time. Schreiber, who specializes in GM’s midsize truck platform, commuted to Yuma at least once a month between November 2015 and February 2017, with each trip lasting two weeks and involving at least three hours per day on the Saguaro. In short, the Colorado ZR2 spent thousands of hours on the trail.

On one of those runs, sensors were hooked up to the Colorado to evaluate its wheel travel, then those numbers were fed into a computer to design a program for the grounds’ shake table. The Colorado was then strapped to the table, which suspends each wheel on a different articulated pillar. The movements of those pillars would then replicate the motions the test3ZR2 experienced on the trail, but the truck’s limits were so high that alarmed passersby assumed there was a problem with the table, alerting technicians to the supposed malfunction. That’s the kind of testing this truck underwent.

Once we got behind the wheel of the ZR2 at the Saguaro’s trailhead, we were legitimately surprised at the complexities GM engineers built into the course. While we assumed that it would all feel like sandy desert, the Saguaro actually features lots of different terrain, and we could definitely feel the difference when running the trail using the ZR2’s different traction aid combinations (rear-wheel drive, rear-wheel drive with the rear differential locked, four-wheel drive high with the rear axle locked/unlocked, etc.). Each setup excelled at different points of the trail, making it easy to understand how the Colorado should be used in a given type of terrain. Paired with Desert Proving Ground’s new “Dynamite Hill” rock garden that requires spotters, four-wheel drive low, and the occasional hit to the rock slider, our trip to Yuma gave us an excellent idea of what the ‘Rado can do.

With our day in the dust coming to a close, we took a few final photos, recovered our laptops from the security desk, and said goodbye to our hosts. As we hit the road and headed for home, we were greeted with one final artillery shell in the distance, its percussion reverberating in our chests. Was it intentional? A grim reminder of what might happen if we betray GM’s secrets?

Perhaps.