This week, scientists at the Lawrence Livermore National Laboratory (LLNL) released another 62 declassified nuclear explosion test videos on the lab’s YouTube channel. These videos follow an earlier batch of nuclear test videos that were released by LLNL in March.

The videos depict atmospheric nuclear tests carried out by the US between 1945 and 1962. Often, the detonations were filmed by as many as 50 different cameras at different angles, locations, and frame speeds. Over time, the US military racked up some 10,000 films from 210 tests.

The films sat in storage for decades until nuclear physicist Gregg Spriggs was asked to model the effects of nuclear explosions, according to an LLNL press release. Spriggs’ model didn’t correspond to information that was published contemporary with the actual tests in the '50s and '60s, so he started digging out old nuclear test films to reanalyze the explosions and fact-check his data against the manually recorded historical data. In some cases, Spriggs found that the decades-old, manually recorded data was off by as much as 30 percent.

That spurred an initiative to reanalyze all of the nuclear test video footage available in order to confirm that the nuclear yield of each test was accurately recorded. Accurate yield data (that is, data on how much energy a weapon releases) helps scientists certify the nuclear stockpile. “These are devastating weapons, and I hope they’re never used in war,” Spriggs said in the LLNL press release. “But the stockpile has been an effective deterrent for more than 70 years. My hope is that this project can help to make sure it stays viable into the future.”

The video-scanning project has required the expertise of rare film expert Jim Moye, who cleans the films and scans them twice in a professional film scanner in order to capture the most exact gradations of color and black and white possible. Many of the films are already decaying after careless storage, so digitization will help preserve the database for future generations.

The videos released this week include footage of the “Harlem Event,” a 1.2 megaton atmospheric detonation above Christmas Island in 1962. The video below shows how nuclear explosions can result in two distinct flashes of light. The first flash happens when the shockwave is formed, but quickly the superheated air around the blast epicenter “shields the light from inside the fireball,” according to LLNL. “As the shockwave cools to below 3,300 Kelvin, the air becomes transparent and the hot gases begin to show through, creating the second pulse.” Measuring the time between these two flashes, as well as the darkest frame between the two flashes, can help calculate the blast’s yield.

Similarly, the “Bighorn Event” (also a 1962 test over Christmas Island) shows a more distinctive fireball and mushroom cloud. “The speed at which the mushroom cloud rises and the height of the cloud can be used to calculate yield,” LLNL writes.

Other metrics by which researchers can measure test yield is the glow time and speed of growth of the initial fireball created by the explosion. A video of the 1955 “Turk Event” released this week occurred just above the desert floor at the Nevada Test Site.