Grimes and Elon Musk secretly welcome second child Exa Dark Sideræl Musk

Watch below: Grimes and Elon Musk welcome second baby via surrogate

Grimes and Elon Musk have secretly welcomed a baby girl, their second child together.

The couple, who are no longer together, revealed that her name is Exa Dark Sideræl Musk.

Sideræl is pronounced 'sigh-deer-ee-el', but they call her "Y". They call their eldest child X Æ A-II — a boy — X A.I. Archangel or "X" for short.

The news was revealed during a new Grimes' interview with Vanity Fair when her daughter started crying from another room and wouldn't stop.

Grimes revealed that Y was born in December 2021 through a surrogate. She was born just a few months after the musician and the entrepreneur announced their separation after three years together.

The singer explained the origin of the name, saying that 'Exa' is a reference to the supercomputing term exaFLOPS, Dark is 'the unknown', while Sideræl is an Elvish 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 her favourite Lord of the Rings character, the powerful Galadriel, who 'chooses to abdicate the ring'" the article explains.

Regarding her relationship with Musk, Grimes said: “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 went on to say that their relationship was "the best it's ever been".

Grimes also said that she was keen on having more children.

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Musk was previously married to author Justine Wilson with who he shares five children. He has also been married twice to St Trinian's actor Talulah Riley.

Grimes and Musk met in 2018 on Twitter and bonded over a joke she once made about architecture and artificial intelligence.

Not long after their relationship went public, Musk and Grimes attended the Met Gala together and their first child was born in May 2020.

When Musk hosted Saturday Night Live the following year, Grimes joined him a sketch playing Princess Peach while the Tesla founder was dressed as Waluigi from the Mario video game series.