What are megatsunamis and how do they happen?

Last summer a huge megatsunami swept through a deserted area of Alaska in the north of the US.

The gigantic wave was caused when part of a mountain crumbled into the sea at Tracy Arm fjord, dropping 64 million cubic metres of rock - the same as 24 Great Pyramids - into the water.

The incredible force of that rock created a massive wave that reached almost 500 metres tall - the second tallest wave ever recorded.

The event happened in the early hours of the morning, in a remote area.

Now a team of scientists have shared new, specialised research into what causes these megatsunamis and if they are happening more often.

Dr Steve Hicks, a scientist at University College London who is part of the research team, explained to Newsround what a megatsunami is.

"A megatsunami is typically a wave that is more than 50 m high.

"Normal tsunamis in the ocean are caused by earthquakes below the sea floor where the sudden up-and-down movement between two blocks of rock (a geological fault) suddenly moves a large amount of water above it."

"Megatsunamis are typically caused by landslides - a sudden collapse of rock within tens of seconds.

"It's a bit like dropping a large object in your bathtub - a wave "runs up" along the side of the bath and maybe even over the top of it."

Megatsunamis "typically happen in narrow bodies of water, such as fjords." he said.

The biggest megatsunami was in the 1958, also in Alaska at Lityua Bay, and was over 500 metres high.

The scientists found from their new research that there is a connection between climate change and megatsunamis.

"We were able to show how ultimately ongoing climate change caused the retreat - or moving backwards - of a large glacier at the end of a fjord in Alaska (Tracy Arm)." Dr Steve said.

"The glacier retreated 500m in the weeks before the landslide, leaving a large mass of rock exposed and unsupported." he said.

As part of their research the team visited the area in Alaska, as well as looking at seismic and satellite information to try to understand the chain of events that created the megatsunami and trace the height of the wave.

Dr Bretwood Higman, an Alaskan geologist who was also part of the team, said there is little doubt that megatsunamis are increasing.

"At this point, I'm pretty confident that these are increasing not just a little bit, but increasing a lot," he said.

"Maybe in the order of 10 times as frequent as they were just a few decades ago."

As a result of their research the scientists have said they would like to monitor more parts of the world that might be vulnerable to megatsunamis.

"Using a combination of real-time data from satellites and seismological sensors, we would like to identify areas of the world which may host a large landslide in the future." said Dr Hicks said.

Scientists would then look at tracking the events that lead to megatsunamis, perhaps giving clues before these events happen.

"One particularly interesting result of our study is that, within the landslide mass that eventually collapsed in Alaska, we detected hundreds of tiny earthquakes in the days to weeks beforehand, giving us hope of developing early-warning systems."