As water freezes, it expands by nine per cent and will force its way out of almost any container. But if you trap it in an ultra-strong canister, a denser type of ice forms. Under normal pressure conditions, water molecules below 0 degrees Celsius (32 degrees Fahrenheit) hold each other at arm’s length to form hexagonal crystals just like snowflakes. This is ice Ih (pronounced ‘one h’), which makes up virtually all the ice on Earth. If there’s no space to expand into, the pressure inside the container will soar to up to 2,000 times normal atmospheric pressure, smashing glass bottles, tin cans and pretty much anything else. Find a container that can withstand the pressure, though, and the water will eventually crystallise into a tighter arrangement, forming pyramid-shaped crystals; this is called ice VI (ice six). By freezing water in a variety of extreme pressure and temperature conditions, researchers have created 16 different types of ice so far.
Answered by Alex Cheung.