Heavy water

Heawy water, chemical symbol D₂O or ²H₂O water with the isotope deuterium replacing normal hydrogen. Heavy water has many uses in nuclear engineering, especially as a moderator.

It was a critical material in the Second World War, and a dramatic raid by the Norwegian resistance destroyed the only heavy water factory available to Nazi Germany, following it up by sinking the ferry that was carrying the last of the production to Germany.