What started off as an “adventure” to go to California to “swim in the ocean” for two US girls, ended in a minor accident and serious damage to two vehicles.

The incident happened on Wednesday morning in Utah - when surprised officers from the West Valley City police department responded to an accident on what appears to be a major road.

Upon arrival, police spotted a stationary truck which had crashed into a red vehicle - later identified as a Chevy Malibu - but they were in for a shock when they realised that the driver of the Chevy was a nine-year-old and the passenger her four-year-old sister. 

According to police, the young girl and her sister “apparently snagged the keys to the family car while their parents were sleeping and set out on their own summer adventure”.

“The girls drove about 10 miles (16km) from their home in West Jordan — via a highway and a freeway — before exiting, hopping a median and slamming head-on into a semi truck. The car suffered serious damage. Even the semi had to be towed!

“Thankfully, though, no-one got hurt, we suspect [largely] because those little girls at least remembered to wear their seat belts!," the department said on its Twitter page.

When the girls were asked what prompted the drive, they told officers they were headed to California because they wanted to “swim in the ocean”.

According to the BBC, the girls' “horrified” parents didn't know they were missing until police alerted them.