Trapped in the rigid structure of diamonds formed deep in the Earth's crust, scientists have discovered a form of water ice that was not previously known to occur naturally on our planet.

The finding, published Thursday in Science, represents the first detection of naturally occurring ice-VII ever found on Earth. And as sometimes happens in the scientific process, it was discovered entirely by accident.

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. (...)

http://www.latimes.com/science/sciencenow/la-sci-sn-water-in-diamonds-20180308-story.html

HN discussion https://news.ycombinator.com/item?id=16552303

Phase diagram of water: https://upload.wikimedia.org/wikipedia/commons/0/08/Phase_diagram_of_water.svg

Famous science-fiction novel about the polywater: Cat's Cradle, featuring a fictional Ice-IX:

Ice-nine is a fictional material that appears in Kurt Vonnegut's novel Cat's Cradle. Ice-nine is described as a polymorph of water which instead of melting at 0 °C (32 °F), melts at 45.8 °C (114.4 °F). When ice-nine comes into contact with liquid water below 45.8 °C (thus effectively becoming supercooled), it acts as a seed crystal and causes the solidification of the entire body of water, which quickly crystallizes as more ice-nine. As people are mostly water, ice-nine kills nearly instantly when ingested or brought into contact with soft tissues exposed to the bloodstream, such as the eyes or tongue.