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The Moon's surface was once an ocean of hot, slushy magma.

That's according to some new research by the University of Cambridge which says that the Moon's bumpy surface could be the way it is because of a process called crystallisation.

According to the theory, crystals remained suspended in liquid magma over hundreds of millions of years as the lunar 'slush' froze and solidified.

The most widely accepted theory of how the Moon formed is the 'giant-impact' theory.

It states that the Moon was formed during a collision between the Earth and another small planet, about the size of Mars. The waste from this impact collected in an orbit around Earth to form the Moon.

As a result of the crash, the Moon was very hot and formed of slushy liquid magma which then solidified over hundreds of millions of years.

In 1969, the Apollo 11 mission saw humans land on the moon for the first time.

During this mission, the astronauts collected rock samples from the Moon in an area called the lunar highlands.

The lunar highlands are the heavily cratered regions of the Moon and they are made up of rocks called anorthosites.

Anorthosite rocks formed early in the history of the Moon, between 4.3 and 4.5 billion years ago.

Similar anorthosite rocks, formed through the magma becoming solid, can be found in ancient magma chambers on Earth.

So as there is loads of anorthosite on the Moon, experts say it's most likely that there was once a huge global magma ocean there.