The was an incredible rat rod of a plane—heavy, big and extraordinarily powerful—and it indirectly led to the development of Western planes like the F-15 Eagle. In the 1970s, the United States Air Force didn’t know much about it, until a Soviet pilot named Viktor Belenko decided to defect and landed one in Hakodate, Japan, 40 years ago today.

The fascinating story of how we learned about the MiG-25, as recounted , starts with spy satellite footage that the West picked up, which showed a mysterious new Soviet jet. It had huge wings. It looked fast, from the blurry photographs we had of it. Nobody knew anything about it.

It was, for all intents and purposes, a ghost.

Meanwhile, Soviet citizen and fighter pilot Belenko’s life was falling apart at home, a divorce looming. He also began to question the entirety of the Soviet system, and if the West was really as bad as he’d been told.

After realizing that the huge, new MiG-25 that he was being trained to fly was his ticket out, Belenko started plotting. The day he flew out for training—September 6, 1976—he also had a full tank of fuel and a route.

From the story:

He broke formation, and within a few minutes he was over the waves, heading towards Japan.To evade both Soviet and Japanese military radar, Belenko had to fly very low – about 100ft (30m) above the sea. When he was far enough into Japanese airspace, he took the MiG up to 20,000ft (6,000m) so it could be picked up by Japanese radar.

The surprised Japanese tried to hail this unidentified aircraft, but Belenko’s radio was tuned to the wrong frequencies. Japanese fighters were scrambled, but by then, Belenko had dropped below the thick cloud cover again. He disappeared off the Japanese radar screens.

A report after the defection captured the harrowing scene of Belenko’s landing in Japan:

The pilot fired two warning pistol shots to keep airport workers away from the plane and then asked the police to cover the plane with a canvas because it contained military secrets.

The Soviet pilot has been identified as Viktor I. Belenko, a lieutenant of the Soviet Air Force. The pilot spent the night at a secret place on the outskirts of Hakodate. The MIG‐25 was heavily guarded throughout the night by the police.

But it was really in the aftermath of Belenko’s flight that the full implications to hold, as the BBC notes:

Japan only really knew what they were dealing with when the MiG made its surprise landing. The Japanese suddenly found themselves with a defecting pilot – and a fighter jet that had so far evaded Western intelligence agencies. Hakodate’s airport suddenly became a hive of intelligence activity.

The CIA was scarcely able to believe its luck.

Before, the idea that the Soviets potentially had a super maneuverable and ridiculously fast secret fighter jet was highly worrisome to the West.

It was only after Belenko’s defection that the MiG-25 was taken apart and more was learned about it, like how it had more engine than made sense, and that if it went above Mach 2.8, the engines would literally start turning themselves inside out because “the force exerted by the compressors would be so huge it would start sucking up parts of the engine.”

It was more of an interceptor and a reconnaissance platform than a fighter, with high wing loading and relatively poor handling qualities.

The Foxbat, which so many people had been so nervous about, turned out to a big and heavy plane that wasn’t particularly useful in combat. Because it needed so much fuel, it had a very short range. It was only fast in a straight line. After it came the MiG-31, an updated variant with increased capability.

Belenko eventually became a U.S. citizen and an aeronautics engineer and consultant to the U.S. Air Force. Once inspection of the MiG-25 was finished, it was partly put back together and shipped back to the USSR in crates. The BBC notes that the Japanese then sent the Soviets a $40,000 bill for shipping costs and for the damage Belenko caused to the Hakodate airport when he landed the giant plane.

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