Ben - We've had an email question from Jim Irvin and he wants to know if you can compress water into a solid. Now obviously you can turn water into a solid just by taking heat away, but can you just press it hard enough to make into a solid structure? Dave what do you think?

Dave - The simple answer is, yes you can. You'd need a ridiculous amount of force, but it is possible. When this happens a different form of ice is formed, called ice IV, which is a different crystal structure to conventional ice.

Ben - Conventional ice has a greater volume than liquid water; so compressing water in order to turn it into conventional ice, you'd need to somehow compress it into a solid and allow it to expand?

Dave - Yes, so it just wouldn't work if you tried to produce conventionally-structured ice.

Ben - Wouldn't work?

Dave - No; but if the ice forms in a different crystal structure that doesn't have a larger volume, which you can achieve with a pressure of about 2 Gigapascals - or about 20,000 atmospheres, which is the same pressure you get under 20 kilometres of ice or water - then you can produce ice at normal room temperature.

Ben - Are there any bits of the world where water is under that sort of pressure, 20 kilometres down deep in some of the deep ocean ridges? And if so why doesn't it form the ice structure?

Dave - Twenty kilometres is about twice as deep as the deepest parts of the ocean, so probably not; though there is no reason why, on other planets with a deeper ocean, you couldn't get this effect.

Ben - So water seems like such a simple thing, such a simple everyday thing, but actually it's fascinating stuff, isn't it?

Dave - It's absolutely bizarre stuff!