Researchers mapping the sea floor near Australia's Great Barrier Reef say they've made an ominous discovery.
They've found a giant slab of sea-floor that's in the early stages of collapse, the ABC reports.
They say it'll break off one day and when that happens it could trigger a tsunami capable of hitting the North Queensland coast.
Marine geologists at James Cook University say it is only a matter of time before the slab, known as the Noggin Block, collapses.
Dr Robin Beaman says the slab is the remains of an underwater landslide deep in the Great Barrier Reef.
"It's actually up on the top of the continental slope in about 350 metres of water," he said.
"This block, which is about one cubic kilometre ... is in the very slow, early stages of starting to break away from the edge of the Great Barrier Reef."
He says the Noggin Block is stable at the moment, but warns a collapse could be catastrophic.
"The only thing that we could find that would trigger a block breaking away on that scale [would be] a very large earthquake in the near vicinity. That is very unlikely," he said.
"If it were to break away catastrophically, that is break away really quickly, what that would do is it would create a surface wave above it. It would actually cause a tsunami.
"That tsunami would travel across the Great Barrier Reef, it's about 70 kilometres offshore, and it would impact the local area, the North Queensland area."
It would take about an hour for the tsunami to hit nearby coastal areas like Mourilyan Harbour and Clump Point.
Dr Beaman and his colleagues hope use their sonar mapping technique to find more underwater areas at risk of collapsing.
"We should be aware that these things exist," he said.
"We don't really know when such a block might collapse. All I can say is sometime it eventually will."
The research is published in the journal Natural Hazards.