Ice Seven!

Naturally-formed Ice Seven finally observed on the Earth!:

Scientists believe that ice-VII may be found in great abundance in the solar system, perhaps in the interior of ice moons like Enceladus and Europa, or as part of the ocean floor of Titan. But they did not think it could naturally occur on Earth.

The pressures ice-VII requires to form can be found on our planet, but they exist only deep in the mantle where the temperature is too warm for this form of ice to be stable.

...What's special about inclusions in diamonds is that the material entrapped within them remains under the same pressure as it was during the time it was encapsulated.

"The diamond lattice doesn't relax much, so the volume of the inclusion remains almost constant whether it's in the Earth's mantle or in your hand," Tschauner said.

...However, as the authors discovered, diamonds can trap small bubbles of extremely dense pressurized water when they form. Then, as the diamond moves up through the mantle, the water inclusion is subjected to cooler temperatures while remaining under the same pressurized conditions. In that very specific case, ice-VII can occur.