Between 1945 and 1962 the U.S. conducted more than 200 atmospheric nuclear weapon tests and captured the detonations on film. Multiple cameras capable of recording 2,400 frames per second covered each blast, creating a highly technical record of the U.S. nuclear arsenal—and a visual deterrent against its use.

At that time physicists analyzed the roughly 10,000 classified movies to understand how the weapons worked and calculate their effects in a potential war scenario. After the U.S. and other nations banned atmospheric testing when they signed the Limited Test Ban Treaty of 1963 and made some of the data in the films public, the recordings could be declassified—but most remained secret and stored away in government vaults. Physicists had to design computer simulations, tested against those mid-century analyses, to “predict what would happen if a weapon went off,” says Greg Spriggs, a nuclear weapon physicist at Lawrence Livermore National Laboratory, who is leading the project to scan and declassify the films. “We’re reanalyzing the films and trying to get benchmark data: What’s the shock wave as a function of time? What would be the thermal blast? How high did the cloud go? What were the cloud’s dimensions?” he says.

The information will help physicists validate their computer simulations with more certainty and assure the bombs would go off as advertised. “We don’t want to give the military something that, if they have to use it—which we hope they don’t—they’re not dropping a dud on the enemy so the enemy can pick up that weapons-grade material and turn it against us,” he adds.

Over the last five years Spriggs and his team of film preservationists have located about 6,500 films, scattered among the government’s high-security vaults and in varying degrees of decomposition. They have scanned approximately 4,500 of these, reanalyzed between 400 and 500 and declassified about 750. In March the first declassified batch was uploaded to YouTube. “They are not the same films that most people are used to seeing,” says Alex Wellerstein, a historian of science at the Stevens Institute of Technology who is not involved with the project. “They show a remarkable amount of technical effort that went into representing the very difficult to capture—because it is so extreme—phenomenon of a nuclear explosion.”

Here are four films that reveal subtle clues about each historic blast:

FILM #1: Operation Dominic—Housatonic 120256

In this film, a device airdropped from about 12,000 feet above the Pacific detonates on October 30, 1962. The film shows the two characteristic light output pulses that are seen only in nuclear weapon blasts, which correlate with the device’s yield (the amount of energy given off from the explosion).

The first pulse occurs when the shock wave forms and light scatters around the blast. As the shock wave–pushed air cools, the temperature at the wave front drops and the light dissipates to its darkest point—called the minimum, or T Min—when the temperature has reached 3,300 kelvins. Then, the air becomes transparent and the hot gases inside the fireball are able to escape, producing the second light pulse. At its brightest point, it is called the maximum, or T 2 Max.

By measuring the time from the detonation to the T Min and the T 2 Max, scientists can calculate the weapon’s yield. “In this film there are probably four or five different ways a scientist could analyze what the yield of that device was—and that’s why these films are so important,” Spriggs says.

FILM #2: Operation Teapot—Tesla 28617

Tesla was a relatively low-yield shot of seven kilotons, dropped from a 300-foot tower at the government’s test site in Nevada on March 1, 1955. When the shock wave hits the ground in the video, it kicks up the dry desert soil and “it makes a nice little dust cloud,” Spriggs says.

After the second pulse of light reaches its maximum, the bright white fireball forms the cap of the mushroom cloud. The shock wave moving away from the blast disturbs the soil, and as the dirt is sucked into the atmosphere, it forms the cloud’s stem. Physicists can find the yield of the device by measuring how fast and how high the mushroom cloud rises, Spriggs says.

FILM #3:Operation Hardtack I—Nutmeg 51538

The Nutmeg test was detonated from a barge tethered to Bikini Atoll in the Pacific on May 21, 1958. In this film the initial blast and scattering of light are visible but high humidity obscures the fireball itself. As the shock wave travels away from the blast, it changes the atmospheric pressure and creates a low-pressure trough in its wake. Condensation collects in the trough and forms a so-called Wilson cloud—which resembles a stack of puffy white rings—hiding the initial fireball. When the low-pressure trough returns to atmospheric pressure, the Wilson cloud dissipates and the glowing fireball and mushroom cloud are revealed. “The glow time of the fireball at the center of the Wilson cloud and the formation of the Wilson cloud are clues to the weapon’s yield,” Spriggs says.

FILM #4: Operation Dominic—Bighorn 110762

The Bighorn device was dropped over the Pacific Ocean from a height of around 12,000 feet on June 27, 1962. Thanks to the high altitude, the detonation occurs above the humid layer of the lower atmosphere. The bright light erupts from the second pulse, and then the gases start to escape. As the shock wave propagates downward, it flows through the moist air and forms a Wilson cloud in the low-pressure trough, underneath the fireball. The shock wave then bounces off the ocean’s surface and flattens the bottom portion of the fireball’s gases. “This was up high enough, you still had a Wilson cloud, but it was down in the lower part as the shock wave went around. It did not obscure the early cloud formation of this particular detonation,” Spriggs says, adding, “That’s a cool film.”