It really got to me. It's dark (I'm using enb drivers). I came in through Alftand with trusty Lydia in tow as always to watch my back. I started off to my right and found swam in a pool until I found CRIMSON nirnroot! Lydia wasn't liking the water so I headed back to the start and went the other way. Killed a falmer and found more crimson nirnroot. There was a locked gate with a level behind it but I didn't see how to get in so I just followed the wall looking for more nirnroot.
I followed the wall until I came to an exit labelled armory and thought I should finish up Black Reach first so I started heading back towards where I came. Then I looked at the map to get my bearings... Wow this place is kind of big I can't even see where I started. I keep going collecting nirnroot and wandering a bit since a lot of rocks and cliffs inhibit travel. I'm lost. Lydia is gone. I don't know how long she has been gone but it's not the first time and she usually reappears so I keep going.
I find a city. There is a city in this underground cave and I had to look around to find it. That's how big this place is. After mostly peaceful exploring I'm being attacked on all sides by about 10 humans the falmer have enslaved and falmer themselves. Why are they so faithful to their masters? Where is Lydia?
I enter the council chambers thinking Lydia might find me there. I'm jumped by 5 more slaves and falmer. I jam myself in a crevice desperately fighting them off. Lydia isn't coming. I go back out to Black Reach just to try finding my way out.
After another half hour of wandering I find the building directly across from the way I entered and decide to check it out. The building is filled with alchemy ingredients and the corpse of Sindarion who'd apparently set up here to research nirnroot. His notes mentioned needing 30 crimson nirnroot for study and someone to contact. I'd found 13 after about an hour and a half. Black Reach was at least double what I'd already explored. Now I really did leave.
Lydia was waiting outside. I was filled with relief. If something had happened to her I probably never would have been able to find out what in such a large place. With that burden off my mind I went back in.
This time I made sure I could hear Lydia's faithful step behind me at all times. Some hills she didn't like and I had to go around. We cleared out area after area with the only reward at the end a single crimson nirnroot but I was determined to finish this place.
Hours passed. I found 4 or 5 elevators to the surface spanning miles of the world. More exits seemed to lead to other dwarven ruins but could not be opened from this side. I cleared a dozen buildings. There was another city with a high throne looking out over the surroundings. I don't think I could see a quarter of the way across the area. I found 30 crimson nirnroot. Then 40. It was time to get out of here. My map showed I still had not been everywhere but areas I'd been had been disappearing off of it possibly because it was too big to keep track. I headed for the main reason I'd come here in the first place.
The rest is another story.
So yeah. Black Reach is big. Bigger than some games. Like from Dawnstar to Winterhold square, plus attached areas which probably double that. I can't remember if the Dwarven ruins in Morrowind all connected or not. Anyway Black Reach gets its own topic.