 All filled ammonia wagons were stored indoors in a tunnel under heavy guard.On 16 November 1943 the United States Army Air Forces bombed the hydrogen plant; the attack killed 21 civilians but failed to damage the heavy water plant as it was located under seven stories of reinforced concrete.The Germans decided to cancel production of heavy water at Rjukan and move the remainder of the potassium hydroxide— from which the heavy water was distilled – to Germany.