"Dad, what do you do there?" "Nothing much, just flying planes."

When Pilot Zhan Zhiqiang returned home and answered his daughter's questions, his answers were always brief and consistent. One day, decades later, Zhan Zhiqiang suddenly gave his daughter another answer: he had worked with Qian Xuesen and was one of the people who had crossed through the mushroom cloud.

Watch "The Man Who Crossed the Mushroom Cloud" Episode 3: "The Silent Oath" together↓

Crossing the Mushroom Cloud: A Pilot's First-Person Perspective on Sampling Revealed. Zhan Zhiqiang was a pilot of the 13th Division of the Air Force. In a certain month and year, he took on a special mission, traveling across the country with his comrades to transport various mysterious cargo to Lop Nur. "There was a special escort on the plane, and after loading a few boxes, everything was sealed tightly." Zhan Zhiqiang had no idea what these mysterious cargo were until the earth-shattering explosion at Lop Nur on a certain day and month, when he realized that what he had been transporting were components of China's first atomic bomb. It was also on this day that Zhan Zhiqiang's crew took off from Lop Nur, delivering samples from the atomic bomb explosion to Beijing for testing.

After the New Year, the Zhan Zhiqiang crew took on another important mission: sampling through the mushroom cloud after a hydrogen bomb explosion. On the day of the successful detonation of China's first hydrogen bomb, the Zhan Zhiqiang crew, waiting in the designated airspace, quickly entered the hydrogen bomb mushroom cloud in their Il- transport aircraft. Zhan Zhiqiang recalled: "When the ground said 'detonate,' we had to quickly pull down the front window, which they called 'blind flying,' but we could still see a bit through the cracks. It looked like a big red sun, and inside the plane, it was like plowing through clouds, turning around four or five times."

Daughter only learned decades later that her father, a pilot, had worked with Qian Xuesen. Zhan Zhiqiang said, "See it, remember it, keep it to yourself, and take it to the grave." From transporting atomic bomb components to delivering nuclear explosion samples, and even personally flying into the mushroom cloud to collect samples, these missions were deeply hidden in his heart, never mentioned for decades. Daughter Zhan Ying recalls, "Every time he came back, I would surround him and ask, 'Dad, what were you doing there?' He would smile and say, 'Nothing much, just flying planes.'" Zhan Zhiqiang's wife passed away without ever knowing what he had done back then. The record of merit in the archives simply summarized Zhan Zhiqiang's glorious past with a single sentence: "Awarded a First Class Collective Merit in the construction of national defense modernization."

Until that year, when Zhan Zhiqiang saw the news of the passing of Qian Xuesen's wife in the newspaper, he shared the declassified past with his daughter. His daughter, Zhan Ying, recalled, "He said he worked with Qian Xuesen, and I was very surprised at the time. I asked him, 'When did you work with Qian Xuesen?' He said, 'During the time they were developing the atomic bomb... He hadn't told us about it in recent years.'"

The development of nuclear weapons is a matter of the highest state secrets. The word "secrecy" has become the bottom line that every participant in nuclear tests must adhere to. Even years after the declassification, Zhan Zhiqiang, with his white hair, only shared a few words about it with his family on an ordinary day. Many others took the secret to their graves without ever revealing it to anyone.

From year to year, China conducted nuclear tests, and in many of these tests, due to the lack of drones at that time, the only option was to have brave pilots fly through the mushroom cloud to collect samples. These pilots, who did extraordinary things while remaining anonymous, made significant contributions to China's nuclear weapons program. Salute! Remember!