You have to climb behind the waterfall to confirm it, but it is definitely same large door you saw on fire in the central room. On this side, it is soaked from the spray of the waterfall and smoldering, producing the clouds of grey smoke that pour out, but with your visual acuity you can still make out the details. It is hot and a little hard to breath in this tunnel, though, and the door blocks the end.

You could probably kick or push open the door from the side without getting burnt if you did it quickly.