"The Lancasters attacking Valentin each carried a single large earthquake bomb – seven carried the 5 ton 'Tallboy', thirteen carried the 10 ton 'Grand Slam'. Two 'Grand Slam's hit the target and penetrated about half-way through the 15-foot (4.6 m) thick ferrous concrete roof before exploding. The explosions blew large holes in the remaining thickness of the roof and brought down around 1,000 tons of debris into the chamber below.[8] Workers who were inside the bunker at the time survived, as the bombs did not penetrate the roof before detonating.[8]"
"The Lancasters attacking Valentin each carried a single large earthquake bomb – seven carried the 5 ton 'Tallboy', thirteen carried the 10 ton 'Grand Slam'. Two 'Grand Slam's hit the target and penetrated about half-way through the 15-foot (4.6 m) thick ferrous concrete roof before exploding. The explosions blew large holes in the remaining thickness of the roof and brought down around 1,000 tons of debris into the chamber below.[8] Workers who were inside the bunker at the time survived, as the bombs did not penetrate the roof before detonating.[8]"
https://en.wikipedia.org/wiki/File:U-Boat_Pen_Grand_Slammed....
https://en.wikipedia.org/wiki/File:Bremen-Farge,_Royal_Air_F...