As we all know, if you go to blackreach and shout at the glowing light an echo is heard, then a dragon called Vulthuryol flies over. 

Perhaps one of the most debated things about the dwemer (between height and how they got the kelle) is how they got Vulthuryol.  

Any ideas?

I think you're talking about Vulthuryol - Volsung is one of the Dragon Priests.

This topic came up on the community wall one time, and we arrived at a few possibilities. One was that, in some ancient era, there existed an entrance to Blackreach large enough for a dragon to fly through. The dwemer lured him down and trapped him there.

Or magic. The dwemer had a lot of it at their disposal, and I don't think it'd be beyond their ability to transport a dragon underground.

One of my ideas was, if they can harness and read an elder scroll, a dragon can't be hard. And yeah, I meant vulthuryol

I think it's actually Vulthuryol. "Dark Overlord Fire." Volthuryol would be "Horror Overlord Fire" (which actually sounds way cooler, now that I think about it).

Here's an interesting theory I came across:

Its highly likely the dragon willingly went into Blackreach, all as part of a plot to keep the Falmer in check.

Just look at the dragons name, Dark Overlord Fire.

He's the nasty warden that keeps the prisoners in check, prisoners who have a specific racial weakness to fire.

paarthurnax

This topic came up on the community wall one time, and we arrived at a few possibilities. One was that, in some ancient era, there existed an entrance to Blackreach large enough for a dragon to fly through. The dwemer lured him down and trapped him there.

Or magic. The dwemer had a lot of it at their disposal, and I don't think it'd be beyond their ability to transport a dragon underground.

Writers' license solves everything ^_^

Uh . . .

If I had to guess, I'd imagine that long ago Blackreach/Alftand (and a lot of the other Dwemer ruins, for that matter) was a lot bigger, considering the size of the machinery and stuff. Some sort of massive hallway that is now completely encased in glacier, or something. Heck, maybe if all the gates and pillars and stuff were cleared out you could even fit a juvenile dragon down one of the litfs, either through negotiation or a trap.

Or, who knows, maybe you could just use an Elder Scroll as a portal the same way you can use one as a time machine. Maybe the Dragon scroll has a twin that manipulates space instead of time? Dialga and Palkia

paarthurnax

Here's an interesting theory I came across:

Its highly likely the dragon willingly went into Blackreach, all as part of a plot to keep the Falmer in check.

Just look at the dragons name, Dark Overlord Fire.

He's the nasty warden that keeps the prisoners in check, prisoners who have a specific racial weakness to fire.

I suppose since dragons were made to dominate, a dragon being handed an entire race of mer to rule, or at least outrank, would be a very appealing concept.

Perhaps he came to be the same way as Nahagliiv. He had some kind deal with the dwemer.(perhaps for the war with the Chimer) If the race of dwarves suddenly dissapeared, it may have been difficult to get himself out? In attempting to escape, he knocked something over that blocked his path and trapped him in until the dragonborns shouts unrelenting force at that thingy. This thingy for some reason opens the path, (a switch?) and allows Vulthuryol into the main chamber. By this point he doesn't care about life and gladly gives his soul for the dragonborn to devoir. 

The End.

I like that one. It fits what Paarth quoted earlier, more or less, and could explain why it's still down there even though you realistically don't need to keep the Falmer in check from anything anymore - it knocked something over, and given how unweildy dragon arms can be, couldn't really do anything about it.