Scientists have discovered thousands of new mountains, all over 1.5km high, that were previously unknown on Earth.
The huge number of new mountains haven't been hiding in plain sight (lurking behind bigger mountains, say) – they're at the bottom of the ocean.
The researchers used radar satellites that indirectly detect tiny variations in the gravitational pull of Earth to create a more accurate map of the seafloor than has ever been produced, and have published their results in Science magazine.
We still know more about the surface of Mars than we do about the floor of our own oceans.
The satellites in question (CryoSat-2, which belongs to the European Space Agency, and NASA's Jason-1) don't directly detect the mountains on the ocean floor – instead they detect the mountains of water that gather around them.
Because Earth's gravitational pull is slightly stronger where there is a large mountain, water is attracted towards them – meaning the ocean's surface is slightly higher where there's an undersea mountain, and lower where there's a trough. These differences are too small to see with the eye, but satellite altimeters can spot them.
The new techniques have allowed the team to detect undersea mountains as "small" as 1.5km high – an improvement on the previous best of 2km.
"That might not sound like a huge improvement," lead researcher Professor David Sandwell of the Scripps Institution of Oceanography told BBC News, "but the number of seamounts goes up exponentially with decreasing size. So, we may be able to detect another 25,000 [mountains] on top of the 5,000 already known."
In addition to the new mountains, the maps have revealed a huge number of ridges and trenches in greater detail than ever before. Another team member, Dietmar Muller of the University of Sydney, told The Guardian: "The disappearance of Malaysia Airlines Flight MH370 earlier this year has heightened global awareness of the poor knowledge of our ocean depths."