On survival in death toll the level "The Drains" there is a big box.

When you walk up ahead move the white canister and put it far away from the big box, then climb up the ladder.

Then take a pipe bomb, throw it under the big box, then it will fly up the ladder.

There will be a big hole in the wall, get on the box then run to the safe room.

Don't forget to call the bell!