Decommissioned lookout Bunker stations here in the cliffs over looking the sf golden gate have extremely deeply sunken thick concrete buried barracks. we turned off the flash lights and it's the darkest I've been in and absolutely quiet with a group of 30+ kids and chaperones. Had better ears back then so no ringing ears. And sunken structures have the benefit of about 65° stable temperature compared to the cold outside air
It was comforting