Donkey Kong Country: Level 27: Oil Drum Alley

The fifth area of DK Island is likely one that didn't exist before King K. Rool stole the Kongs' bananas, because this area his his own toxic industrial site of Kremcroc Industries Inc.!

The first level takes place in one of his factories. The oil drums themselves are usually already set on fire, but they can be destroyed with TNT.
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There are four Bonus Rooms in this level, and boy, you have to REALLY know what you're doing to get to one of them!:

The first Bonus Room is near the beginning. First, use a rope to fall from a great height to open a black hatch in the ground get a TNT Barrel. Then, take that barrel to blow up a flaming oil drum to reac the room. This is another room where you have to use Barrel Cannons to reach a column of bananas, and also a 1-UP Balloon.

The second Bonus Room is found by releasing a TNT Barrel from another black hatch, then take it to a wall. This leads to yet another Stop the Barrel bonus of matching 3 of the same pickup in a row. But...

The third Bonus Room, and read this carefully, now, IS ACCESSED FROM THE SECOND BONUS ROOM! To reach it, you MUST have 3 bananas lined up in the Stop the Barrel bonus. This will spawn a plain barrel that you can then use to bust open the wall to the right. HOWEVER! You have to jump over to the wall and hug as the barrel busts it open so that you will automatically go into it. If you just throw the barrel at the wall, or just walk up to it, the bonus will finish as a victory, but you'll be taken out without any chance of reaching this third Bonus Room.
-- The room itself has you spell out each of the words of "DONKEY KONG COUNTRY", and as you spell more words, you will get more and more valuable Balloons.

The fourth Bonus Room is mercifully less complex. You can find Rambi in one of the fake pits, and you can just use him to ram through a wall to this room. This Bonus Room is yet another one where you use Barrel Cannons to cross a long gap.

This is the ONLY level in the entire game that has one Bonus Room inside of another one. Hopefully this video helped you reach it!