cold deaths
Millions people were rounded up and killed in gas chambers during WWII. This was the easiest way to kill many people at one time. If you were not fit to work, too old or young to, then you would have to die. First they would round up the groups in large numbers and take them underground. There, in a very crowded area they would have to undress. Most of them knew what was coming, but they could not escape. Even the little children and babies went through this process. After they were all undressed, they would be herded to another big room with shower heads on the walls and ceilings. They were told to turn the knobs and take showers. As soon as they turned the shower heads on Carbon Monoxide filled the room. It was a slow and painful death for each and every one of them and there were no survivors.
Corpses of prisoners discovered by Soviet war crimes investigators