New book reveals Elon Musk's secret order near Crimea 1:46

(CNN) -- Elon Musk confirmed that he and his ex-partner Grimes had a third child together, and his name is as unusual as his brothers.

The boy is called Tau Techno Mechanicus, the tech billionaire wrote in a post on X (formerly Twitter) on Sunday.

Musk, owner of the social network X, was responding to a post from the entertainment news account Pop Base, which named his and Grimes' three children as "X Æ A-XII, Exa Dark Sideræl Musk, Techno Mechanicus."

Tau Techno Mechanicus

Circumference/Diameter

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— Elon Musk (@elonmusk) September 10, 2023

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The boy's existence had been kept secret until recently.

Musk and Grimes split in September 2021, but the Canadian singer — whose real name is Claire Boucher — described her relationship status as "very fluid."

In a March 2022 interview with Vanity Fair, she revealed that Exa Dark Sideræl Musk was born by surrogacy in December 2021.

Although the couple apparently had no intention of announcing the birth of their second child, nicknamed "Y," Grimes shared the news after Vanity Fair reporter Devin Gordon heard a baby crying.

Grimes and Musk previously welcomed a son named X Æ A-XII in May 2020.

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Musk, 48, has five other children — twins Griffin and Vivian and triplets Damian, Saxon and Kai — from a previous marriage.

Musk has previously expressed concern about declining birth rates.

"The U.S. birth rate has been below sustainable levels for about 50 years," Musk wrote on Twitter in May 2022.

And he reiterated his position in July of that same year.

"Doing everything possible to help the subpopulation crisis," he posted. "A collapsing birth rate is the greatest danger facing civilization by far."

Elon Musk