Minecraft wiki entry for Exploring_an_ancient_city, associated text: You will first need to go to very deep down underground, deep enough that you reach the Deepslate layer, close to Bedrock. You will want to approach and reach the Ancient City either from the same level or from below, as if you mine into it from above, you run the risk of falling directly into it. If the fall damage does not kill you, the warden certainly will. Once you get to somewhere between Y=-44 and Y=-52, begin strip- or branch mining. This will give you the highest chance of finding a city. The Ancient City will also be more likely to spawn under the new mountain biomes.