In the furnace room, you will see two furnaces (the choice is yours).
Go up the stairs inside and go near your chosen furnace.
Go to the spot closest to the thing that looks like a bucket of coal attatched to the furnace (still on the platform above).
Run off the edge towards the coal thingie, but jump before you fall.
Make sure you land on the coal thingie. When you land there, crouch down and walk into the furnace.
If you lay down, you can still see outside the furnace.
People on the outside can shoot and throw grenades through the cage, but so can you.
They can also only see you though the cage. The only way to get out is to either die or respawn.
Each furnace can fit 5 people at a time.