It only generates inside the Deep Dark, roughly Y=-51, and the Warden that lives there is blind — it hears and smells you instead, which is exactly what you can work around.
An Ancient City only generates inside the Deep Dark biome, with its floor sitting around Y=-51. There's no other way to spawn one, no seed setting that guarantees it, and no way to tell you're near one except by finding the biome first and hoping.
The Deep Dark generates deep underground, in areas of low erosion — in practice, that means it tends to sit beneath mountains and other tall, stable terrain rather than under flat plains. It stays mostly within the deepslate layer, which already puts it well below sea level before you've found anything. There's no shortcut around exploring: head down toward Y=-51/-52 under promising terrain, dig or cave your way through, and the biome itself is unmistakable once you're in it.
You'll know before you see the city. The Deep Dark is dim — there's almost no glow lichen lighting it, just the pale blue glow of sculk catalysts and the occasional patch of lava. The floor and walls are covered in sculk: dark, veined blocks that spread from catalysts placed near where mobs die, dotted with sculk sensors and sculk shriekers. Two ambient music tracks alternate in the biome, both noticeably quieter and more unsettling than anything on the surface. None of that is decoration — the sculk is the tell that an Ancient City might be close, since the structure generates only inside this one biome and the sensors and shriekers show up far more densely around the city itself than out in the open Deep Dark.
The city itself is a sprawling, sunken ruin — corridors floored in gray wool, smaller structures branching off the sides, and a central plaza built around a portal-shaped frame of reinforced deepslate you can't obtain in Survival. Explorer maps exist that point toward one, but they're themselves a rare find tucked away in other structures — not a reliable planning tool so much as a lucky shortcut if you happen to have one.
Everything in an Ancient City is built around one mob you're not meant to fight. The Warden is completely blind — it has no eyes that function — and detects players two other ways instead: vibrations and smell.
Vibrations are picked up across a roughly 15-block radius around the Warden itself, on top of whatever the sculk sensors scattered through the city are separately listening for. Walking, breaking blocks, opening chests, and combat all send out vibrations that a nearby sensor or an awake Warden will register. Smell is the second, separate check: the Warden periodically sniffs the air around it, and that check isn't blocked by sneaking the way vibration detection is.
The practical rule is simple even if the mechanics underneath are specific: sneak, and don't do anything else you don't have to.
The loot is real, not just a reason to justify the danger. Ancient City chests turn up disc fragments (which combine into an exclusive music disc), armor trims, enchanted gear, and — most valuably — echo shards, which don't drop anywhere else in the game. Eight echo shards combined with a compass at a crafting table make a Recovery Compass, which points toward wherever you last died, useful on its own merits well beyond this trip. Some chambers hold specialised loot with their own smaller item pools on top of the standard chest tables. None of it requires fighting the Warden to collect — the entire point of the sneak-and-wool approach is getting the chests open and closed without ever waking one up.
Run, don't fight. The Warden hits hard in melee and can also fire a sonic boom attack that passes straight through blocks and armor, and being caught in its detection range applies Darkness to everyone nearby, making the room even harder to read. It's slower than a sprinting player over open ground, so putting distance and, ideally, a solid path of blocks between you and it is the realistic way out — not gear, not a plan to win the fight. Once you're clear, the Warden eventually loses interest and returns to being dormant, which is the same behavior that let you sneak past it undetected in the first place.
If you're stocking up on other underground finds while you're already this deep, the depth range for an Ancient City isn't far off the range where you'd be looking for diamonds anyway — worth planning the same trip around both.