Billionaire Tesla founder Elon Musk has tried his hand at cars, rockets, tunneling and even hopes to colonise Mars, but one of his most secret ventures has remained hidden at his SpaceX campus.

The controversial billionaire has founded a school, called Ad Astra, which is based in his rocket company's offices in Hawthorne, California.

Ad Astra is one of Musk's most personal ventures. After taking his five children out of school in 2014, Musk founded Ad Astra as a school project dedicated to the next generation of child geniuses.

The project is unlike other schools in the US. Its loose curriculum focuses on projects the entrepreneur is most interested in, from artificial intelligence and machine ethics to robotics classes and coding. In a move that would horrify many parents, it has no room for languages or sport.

Musk founded the experimental school in 2015 to “exceed traditional school metrics on all relevant subject matter through unique project-based learning experiences,” according to a regulatory filing discovered by tech website Ars Technica.

While Musk is normally happy to grandstand about his various projects, Ad Astra, which comes from the Latin meaning "to the stars", is a mostly private venture. It educates children from seven to 14, starting out with a class of eight including Musk's own children. It has since grown to around 40 students made up of gifted applicants and the children of SpaceX employees.