In a nutshell: Apple doesn't have the best relationship with government agencies, but back in 2005, Cupertino took on a "special assignment" to assist the US Department of Energy in building a "top secret" iPod that had a special, hidden function.

Apple software engineer David Shayer recounts the tale in Apple newsletter Tidbits. He writes that fifteen years ago, the director of iPod software informed him that two engineers from the Department of Energy wanted a special iPod built. It turned out that the pair---Paul and Matthew---actually worked for Bechtel, a large US defense contractor to the DoE.

The pair wanted custom hardware added to an iPod that would record data from this hardware to the iPod's disk in a way that wouldn't be easily detected. The device must still work as a music player, too.

"They were careful to make sure I never saw the hardware," Shayer wrote. "And I never did."

Sayer never found out the iPod's intended use but speculates that the agents were "building something like a stealth Geiger counter," which could allow them to detect levels of radiation surreptitiously. They could, for example, search for a dirty bomb or stolen uranium in a city without alarming residents.

Shayer says there were only two other people at Apple who knew about the secret project: the vice president of the iPod division and the senior vice president of hardware. Even CEO Steve Jobs was unaware of it. All those involved in the project are no longer at the company.

"This wasn't a collaboration with Bechtel with a contract and payment," Shayer wrote. "It was Apple doing a favor under the table for the Department of Energy."

The story has been corroborated by former iPod chief Tony Fadell, who was vice president of iPod at the time.

Apple hasn't been as eager to cooperate with government agencies in recent years. It has repeatedly stated an unwillingness to create backdoors in its devices and had a long-running saga with the FBI over its refusal to help unlock the San Bernardino shooter's iPhone.

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