A SECRET “city” hidden behind miles of trees has free gas, cars without license plates, and roads deliberately covered in potholes.

The 100-year-old compound resembles a self-contained community, but only around 200 people reportedly have complete access to everything inside.

The mysterious site is General Motors’ Milford Proving Ground, a sprawling vehicle-testing facility in suburban Michigan.

Spread across more than 4,000 acres, the private complex has almost 150 miles of roads and more than 150 buildings.

It also has its own gas station, fire and emergency crews, medical center, electrical substation, and wastewater treatment plant.

But despite its city-like scale, members of the public cannot simply move in or freely explore the grounds.

Around 4,200 GM employees work at Milford, although access to its most sensitive testing areas is tightly restricted.

A Los Angeles content creator known as Uptin recently entered the compound and said he was stunned by what lay beyond its guarded gates.

The site contains roads resembling public highways, gravel trails, steep hills, off-road courses, and huge stretches of open pavement.

Vehicles being developed at Milford do not need standard license plates because they are driven on private property.

Fuel is also supplied for testing as GM engineers put new cars, trucks, and SUVs through millions of miles each year.

“Engineers will drive more than 200mph over private tracks,” Uptin explained in a video documenting his visit.

“And because it’s a private city, it’s totally legal.”

Rather than repairing every damaged surface, engineers intentionally create potholes and punishing road conditions to test how vehicles cope.

Cars can also be subjected to temperatures ranging from minus 40 degrees Fahrenheit to 130 degrees.

Testing equipment can recreate extreme altitude, humidity, and wind conditions without vehicles leaving the property.

One of the compound’s most striking areas is Black Lake, a 67-acre asphalt test pad equivalent to about 51 football fields.

Its 2.9-mile road course includes 17 turns inspired by sections of famous racing circuits around the world.

Drivers can reach speeds above 150mph along the course’s straightaway, according to GM.

The company purchased the original land in 1923 for just over $100,000 before opening the facility two years later.

Milford began with 1,125 acres, two buildings, and only 5.5 miles of test roads.

It has since grown into the world’s oldest dedicated vehicle-testing facility, positioned near GM centers in Detroit, Flint, Lansing, and Pontiac.

Staff now complete more than 15 million development and testing miles at the site each year.

The compound’s crash laboratory is anchored by 70 tons of reinforced concrete secured to the bedrock below.

GM conducted its first rollover tests at Milford in 1934 and later developed an indoor facility capable of flipping vehicles onto their sides, roofs, or end over end.

Engineers at the site also helped develop guardrails, with more than 130 miles of the safety barriers now installed around the grounds.

Major advances tested at Milford include automatic transmissions, catalytic converters, child seats, crash-test dummies, and driver-assistance systems.

Every Chevrolet Corvette since the sports car was introduced in 1953 has also been tested there.

The facility has drawn comparisons with Ferrari’s Fiorano test track in Italy and SpaceX’s Starbase operation in Texas.

However, Milford remains a working automotive research compound rather than an incorporated city with permanent residents.

Its tight security and wall of surrounding woodland have helped keep much of the activity inside hidden from nearby drivers for generations.

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