Ice-VII which was previously thought to not naturally occur on Earth was found trapped in a diamond. A team of scientists that were looking for an unusual phase of carbon dioxide while they accidentally found it.
Ice-VII is about one-and-a-half times as dense as the regular ice we put in our drinks and skate on in winter, and the crystalline structure of its atoms is different as well.
In normal ice, known as ice-I, the oxygen atoms arrange themselves in a hexagonal shape. In ice-VII these atoms are arranged in a cubic shape.
But water ice has very low compressibility. When it gets subjected to too much pressure, the atoms don’t scooch together. Instead, they rearrange themselves into different patterns.
For example, if you press down hard enough on ice-I, it will transform into ice-II, which has a rhombohedral structure. Increase the pressure once again and the atoms will rearrange themselves into ice-III, then IV, V, VI and VII.
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.