Enter your seed. Every ancient city appears on your real overworld terrain and the panel lists the nearest ones to the center of the map with distance, direction and exact coordinates.
The cities are deep underground (floor around Y=-51). Use the Biome Finder to see the deep dark around them.
The terrain is the same for both editions: since 1.18 one seed gives the same overworld on Java and Bedrock.
Ancient cities land in different spots on each edition, and the map shows the right ones for whichever you pick. Bedrock uses its own random positions and its own deep dark check, which took some digging through the game to get right.
Java positions are built on cubiomes, a re-implementation of Minecraft's world generator, and I checked them against worlds made by real Java and Bedrock servers. If you find one that's off, tell me in the comments with your seed and coordinates and I'll look into it.
Reference: Ancient City Finder
Ancient cities are the most dangerous structures in the overworld: huge deep dark ruins guarded by sculk sensors and the warden, with loot you cannot get anywhere else.
Getting in safely
Sneak everywhere to avoid triggering sculk sensors, place wool over sensors in your path, and never let a sculk shrieker go off three times: that summons the warden, which is far stronger than your gear. Bring blocks to pillar up and out if it comes.
Loot worth the risk
Swift Sneak books exist only in ancient city chests. Echo shards craft the recovery compass, which points to where you last died. Enchanted golden apples, disc 5 fragments and the ward and silence trims round it out.
Where they generate
Ancient cities are placed on a grid of 24 by 24 chunk regions, but only where the deep dark is underneath, which mostly means under mountains and high ground. That is why they cluster in some areas and are absent from others.
How do I find an ancient city?
Enter your seed and every ancient city appears on the map with exact coordinates; the nearest ones are listed with distance and direction. Ancient cities sit in the deep dark at about Y=-51, often under mountains.
What Y level are ancient cities at?
The city floor is at about Y=-51, deep in the deep dark biome. Dig down at the coordinates carefully and quietly: sculk shriekers summon the warden.
What loot is in an ancient city?
Swift Sneak enchanted books (only found here), echo shards for the recovery compass, enchanted golden apples, music disc fragments for disc 5, and the ward and silence armor trims.
Does this work on Bedrock?
Yes. Bedrock places ancient cities in different spots than Java for the same seed, and the map switches to the Bedrock positions when Bedrock is selected.