system
Given the following post, respond as if you're commenting on Reddit:
human
[W40k] Boiling the oceans away?

In the books, many worlds including Terra have had their oceans boiled away through war. Is that really a viable concept? Where would all the water go? Hydrogen and oxygen gas?
gpt
Well, conversion into Hydrogen Fuel was probably one place it went. When you're powering billions of Plasma Reactors the size of office buildings, you go through a respectable amount of fuel, and water is a hell of a lot easier to work with than gas-giants. 

Depending on the weapons used, it's also possible that the oceans were literally boiled, and then the atmosphere blown off somehow. The Imperium has weapons that can do that, so it's not a stretch to think that they were used in the past.