A gigantic cavity almost 300 metres tall and two-thirds the size of Manhattan has been discovered growing at the bottom of a glacier in West Antarctica.

Key points: The size and explosive growth rate of the newfound hole has surprised scientists

The size and explosive growth rate of the newfound hole has surprised scientists It was revealed by ice-penetrating radar deployed by NASA's Operation IceBridge

It was revealed by ice-penetrating radar deployed by NASA's Operation IceBridge If the glacier melts completely, Ocean levels could rise by 65cm

The Thwaites Glacier — which is itself more than twice the size of Tasmania — is currently responsible for approximately 4 per cent of global sea level rise.

The NASA-led study expected to find a few gaps between the ice and the bedrock underneath the glacier, where ocean water could flow in and melt it from below — but researchers said the size and explosive growth rate of the cavity took them by surprise.

They think it would have contained 14 billion tonnes of ice, and said most of that ice melted over the past three years.

"We have suspected for years that Thwaites was not tightly attached to the bedrock beneath it," said NASA's Eric Rignot, one of the study's co-authors.

"Thanks to a new generation of satellites, we can finally see the detail."

The Thwaites Glacier is currently responsible for about 4 per cent of global sea level rise. ( Supplied: Flickr/NASA/James Yungel )

The enormous cavity was revealed by ice-penetrating radar deployed by NASA's Operation Icebridge, a project that started in 2010 to study connections between the polar regions and the global climate.

"[The size of] a cavity under a glacier plays an important role in melting," the study's lead author, Pietro Milillo, said.

"As more heat and water get under the glacier, it melts faster."

Sea levels could rise 65cm if Thwaites melts

Scientists use satellite or airborne instrument data to observe changes to a glacier's flow speed and surface height as they melt over time.

They also pay attention to a glacier's "grounding line" — the spot where a glacier moves off land and starts floating on seawater.

The more the underbelly of a glacier is exposed to seawater, the more likely it is to melt faster.

If the Thwaites Glacier melts completely, it holds enough ice to raise the world's ocean a little over 65 centimetres, and if all the ice in its neighbouring Antarctic glaciers also melts, it would raise global sea levels an additional 2.4 metres.

A big international field project is now being launched to study the glacier, one of the hardest places to reach on Earth, to try to understand what is causing the rapid melt.

Professor Rignot said it was important to build understanding about how the glacier is being affected by being increasingly exposed to seawater.

"Understanding how the ocean melts away this glacier is essential to project its impact on sea level rise in the coming decades," he said.