By Mike McKimm
Science Correspondent, BBC Northern Ireland
It's a landscape no human has even seen. And those who live right beside it had no idea it even existed.
Deep below the sea, off the north coast of Northern Ireland, a dramatic geological mystery has been discovered.
Huge cliffs, vast basins and plateaus, a lake and even rivers have been found. But so far no-one is certain what caused them to end up like this deep under the sea.
The discovery was made when the seabed was being surveyed to update old Admiralty charts, drawn up in the mid-1800s.
Funded by the European Union and backed by the UK's Maritime and Coastguard Agency, a survey vessel has been scanning the seabed along most of the north coast of Ireland, including the seabed north of Rathlin Island.
Marine biologists have been surveying the sea bed
Most of the bottom was largely flat and unremarkable, but as the survey headed east it suddenly came across an unexpected landscape.
For the first time marine biologists could understand what was down there and the scale of it all.
"I'm always very envious of my terrestrial biologist colleagues", said Joe Breen, Head of Aquatic Science with Northern Ireland's new Environment Agency, who has dived the area for years.
"They can go out on land and see where their habitats are. Underwater we've never had that luxury.
"On a dive you can only see about 15 metres so it's like operating in fog. Now, with this survey, we can report on the true extent of the features.
"For the European Habitat Directive, we have to report the extent of our reefs and sandbanks. This will help with the whole concept of marine spatial planning.
"So, if someone wants to put in renewable energy or extract aggregate, we now have a blueprint and can see how they're going to interact and if it's sustainable."
The survey is to update old Admiralty charts of the area
One of the most striking details is a large lake or crater on what was once the top of huge cliffs towering above the plateau below.
The streams and rivers that fed it are still clearly defined.
And that raises one of the mysteries. Why did coastal erosion not obliterate all that detail as the sea slow rose over the land?
Could it mean that some cataclysmic event took place that allowed the sea to overwhelm the land before erosion could begin?
But already the marine scientists are excited about what they've found.
"We can now get a true idea of the true extent of the rare and endangered species and habitats", said Mr Breen.
"We can now see that we have got more of certain features which we weren't aware of - like sandbanks and reefs. The sandbank features in particular are stunning."
The survey is part of a 2m euro cross-border collaboration with the Marine Institute of Ireland. The area covered is a three nautical mile strip ranging from Tory Island off Donegal to Torr Head near Ballycastle.