I experienced that very same thing when I first smoked some DMT. However, like you, I do not know for sure what dose got me there. Me and a friend had some enhanced leaf made with mint, and loaded up my two foot bong with it. We didnt know how strong it was, so we decided to also place a nice sized crystal on top of it. Our intentions were to not smoke it all at once, but that is what happened anyways.
The very first thing that happened upon exhaling the one giant hit, was a tunnel like appeared very much like what you are describing. It lasted for a short time, and then I hit the end of it breaking through a illuminating white light into what I believe was the spirit realm.
This is that experience
https://www.dmt-nexus.me...;t=32122&find=unread Everything published by Gone-and-Back are the mad rantings and ravings of a mind who yearns to be free and thinks he knows what he is talking about. However, these are just delusions made to feel that freedom, because that freedom will never come. Any experiments done are purely figments of the imagination, and are falsified to the highest degree. Nothing should be taken seriously from a crazy mans mind.