I've sunk more hours into Skyrim than I'd admit to a stranger, and the first time I climbed down into Blackreach I actually put the controller down for a second. There's a vast glowing cavern under the whole province, lit by mushrooms the size of trees, and the game never once tells you it's coming. So here's the full picture: how you get in, what you'll find, and why I think it's still Bethesda's high-water mark.
Getting in
The honest answer is that Blackreach is gated behind the College of Winterhold's main questline, specifically "Elder Knowledge." You need to read an Elder Scroll to learn the location of the others, and to do that Septimus Signus sends you down. The clean route is through Alftand, the ruined Dwemer city northwest of Mzulft, where you descend through the collapsed ruin, fight your way past Falmer and chaurus, and reach the Lexicon receptacle that opens the gate to the great lift.
Once you've been down there, you don't have to do the whole crawl again. There are three Great Lifts that punch straight up to the surface, Great Lift at Alftand, Great Lift at Mzinchaleft, and Great Lift at Raldbthar, and each opens a sealed shortcut once you've activated it from below. There's also a fourth way in via the Tower of Mzark inside Blackreach itself, which is where Septimus's whole errand actually pays off. If you're a returning player who just wants the scenery, save your game at one of those lift doors; it's the fastest in-and-out.
The glowing world itself
What makes Blackreach work is restraint. It's enormous, but it's quiet. The light comes from those bioluminescent Crimson Nirnroot and the huge mushroom canopies, and there's an actual sky-mechanism, a fake sun, that the Tower of Mzark can manipulate. You'll find a Dwemer "city," ruins, a Word Wall for the Storm Call shout if memory serves, and a couple of surprises like a derelict tower with a mage you can talk to. The scale plays a trick on you: you keep expecting a boss arena, and instead you get this melancholy, abandoned grandeur. It feels like a place that was lived in and then lost, which is exactly what the Dwemer story is.
Crimson Nirnroot and the Falmer
Two things define the moment-to-moment of being down there. First, the Crimson Nirnroot. It's the red cousin of the surface plant, it only grows in Blackreach, and it ties into the side-quest "A Return To Your Roots" from Sinderion's old lab. Avrusa Sarethi in the Rift wants thirty of them. They chime like the normal Nirnroot, so you'll hear one before you see it, and hunting all thirty is a great excuse to actually explore every corner of the cavern instead of beelining the objective.
Second, the Falmer. These are the Snow Elves' degenerate descendants, blinded and enslaved by the Dwemer and twisted over centuries underground, and Blackreach is basically their kingdom. They run the place alongside the chaurus they farm. If you've read the in-game book "The Falmer: A Study" you'll know the lore, but seeing their camps strung through a Dwemer ruin makes the tragedy land harder than any book does. Bring something with reach; they ambush.
Best location Bethesda's ever built?
So here's my actual question, and I'm genuinely torn. Blackreach is the one I always come back to, but I can think of rivals. Apocrypha in the Dragonborn DLC, the Soul Cairn, even Vivec City's cantons back in Morrowind, or the Imperial City sewers if you're feeling contrarian. What pushes Blackreach over the line for me is that it's optional, unsignposted, and totally self-contained: you stumble into a second world hiding under the first.
But "best single location they've ever built" is a big claim across thirty years of games. If you'd swap in Apocrypha or something from Morrowind, I want to hear the case. What beats Blackreach for you, and why?