The Boiler Room is a stage candidate for SSB4. It is based on the boiler room level in Conker's Bad Fur Day.
The Boiler Room would also be flat, with one or two floating platforms. Every so often, the Big Big Guy (boiler) would turn on and start shooting fire out of its grate, damaging anyone it hits.
How to unlock
This stage can be unlocked by clearing Classic Mode with Conker.
Big Big Guy Battle
The Old Chap