A new biography of the Tesla Motors CEO claims the billionaire and the singer had another kid together under the radar.

Keeping things under wraps! According to a new biography of Tesla Motors CEO Elon Musk, the billionaire and his on-and-off partner, singer Grimes, now have three children together.

The forthcoming biography, Elon Musk, penned by Walter Isaacson, claims that Musk and Grimes recently quietly welcomed their third child, a son named Techno Mechanicus.

The claim was revealed in a New York Times review of the book -- which hits shelves on Sept. 12 -- although no additional information or details regarding the child's birth or age were disclosed in the article.

Musk and Grimes are already parents to two children together -- a 3-year-old son named X Æ A-12 (or X for short), and a daughter named Exa Dark Sideræl, born via surrogate in December 2021.

In March 2022, Grimes was profiled as part of a Vanity Fair cover story, in which she spoke on the meaning behind her daughter's unique name and the nature of her relationship with Musk.

Grimes said of their relationship, "There’s no real word for it. I would probably refer to him as my boyfriend, but we’re very fluid. We live in separate houses. We’re best friends. We see each other all the time...  We just have our own thing going on, and I don’t expect other people to understand it.”

She added that their relationship is "the best it's ever been," saying they plan to have more children in the future. "We've always wanted at least three or four," she said at the time. Although the same day the issue was published, Grimes took to Twitter to explain that she and Musk had subsequently broken up once again.

As for the unique name, Grimes said that Exa is a reference to the supercomputing term exaFLOPS (the ability to perform 1 quintillion floating-point operations per second), while Dark represents "the unknown."

"People fear it but truly it’s the absence of photons," Grimes says. "Dark matter is the beautiful mystery of our universe.” 

As for Sideræl -- pronounced sigh-deer-ee-el -- Grimes describes it as "a more elven" spelling of "sidereal," which means “the true time of the universe, star time, deep space time, not our relative earth time." It’s also a nod to Grimes' favourite Lord of the Rings character, Galadriel. 

Before the latest news of Musk's alleged third child with Grimes, he had already fathered 10 children, including his two others with the singer.

He also shares 19-year-old twins, Vivian and Griffin, as well as 17-year-old triplets, Kai, Saxon and Damian, with ex-wife Justine Musk. The couple, who was married from 2000 to 2008, also welcomed Nevada in 2002 but he died at just 10 weeks old from Sudden Infant Death Syndrome.

He also reportedly welcomed twins with one of his top executives in 2021, just weeks before he and singer Grimes welcomed their daughter, via surrogate. Their names have not been released publicly.