Climbing to the top of Mount Everest is at the top of any adventurer's bucket list - but research suggests it might not actually be the world's tallest mountain.

Everest, located in the Himalayas, stands at 29,032 feet (8,849 meters), and takes around 42 days to summit. Due to the unforgiving conditions, it's also known to have claimed the lives of at least 340 people.

However, two 'secret' mountains have been discovered on the boundary between Africa and the Pacific Ocean, and they're 100 times taller than Everest at a modest 620 miles (1,000km).

"Why have we never seen them?", we hear you ask.

Well, that's because they actually start far below the Earth's surface - approximately 1,200 miles deep down between the core and mantle of the planet.

The mantle is the thickest layer of the Earth and is composed of magnesium-rich rock. It's considered a 'graveyard' for ancient pieces of the ocean floor, and also explains why there's mountains in locations like Hawaii.

The deep-Earth mountains began gaining attention from scientists in 1996, when they were exploring the core-mantle boundary beneath the central Pacific Ocean, and realised earthquakes were slowing down at certain points.

Scientists from Utrecht University estimate that the mountains are at least half a billion years old, but could even date back to when the Earth formed.

"Nobody knows what they are, and whether they are only a temporary phenomenon, or if they have been sitting there for millions or perhaps even billions of years", says lead researcher, Dr Arwen Deuss.

But unfortunately, that does mean you won't be able to climb them.