Install The Lost Cities on your server

The Lost Cities turns the Overworld into an abandoned civilisation of ruined skyscrapers, apartment blocks, roads, highways, bridges and subways, with loot and dangers inside. It runs on the server alone, so players can join without installing it.

Checked against The Lost Cities 26.3-12.0.0 on NeoForge 26.3, October 2026.

What you need

• NeoForge on Minecraft 26.3. The 12.0.0 jar asks for NeoForge 26.3.0.83 or newer. Pick 26.3 in JAR Manager and the panel runs the server on Java 25 for you.
• Nothing else required. Modrinth lists The One Probe as an optional extra.
• Players. The project page lists server-side-only operation as a feature, so unmodded clients can connect, and Modrinth marks the mod unsupported on the client. A player only needs the mod for its in-game configuration screen or if a modpack needs it on both sides.
• Other versions. Modrinth also has builds for Forge 1.20.1, NeoForge 1.21.1, 1.21.11, 26.1.2 and 26.2, and Fabric 26.2 and 26.3. The Fabric builds list two required libraries and are not covered here. The 1.20.1 version is set up differently, through a defaultconfigs folder.

Switch the server to NeoForge 26.3

1. Open JAR Manager, under More in the left rail.
2. Set Server software to NeoForge and Minecraft version to 26.3, then press Apply change.
3. Leave Take a backup first ticked, press and hold Confirm & reinstall, and wait for the console to finish.

Change your Minecraft version or mod loader covers the switch in full.

Install it and pick a profile

Choose the profile before the world is generated. The project page warns that changing it later only affects newly generated chunks and can leave abrupt borders. The page's server steps are written for 1.21.1 to 26.1.2, and the 26.3 source registers the same lostcities-server.toml in the config folder, so they apply here.

1. Open the Mods tab under Content, leave the Modrinth source chip selected, search for The Lost Cities and press Install.
2. Go to Console, press Start, accept the EULA prompt, wait for Done, then press Stop. This start writes the config files.
3. Open config/lostcities-server.toml in the File Manager. In the [profiles] section set selectedProfile to the profile you want, for example selectedProfile = "default", and leave selectedCustomJson empty. Press Save.
4. The first start may already have made a normal world. Open Config Editor and give World folder a new name, or back up and delete the old world, so the next start builds a fresh one.
5. Press Start.

A non-empty selectedProfile turns Lost Cities on in the new Overworld, and an empty one leaves the normal Overworld alone. Profile names are case-sensitive.

Built-in profiles

The mod's source defines these selectable profiles: default, cavern, nodamage, floating, space, biosphere, rarecities, onlycities, tallbuildings, safe, ancient, wasteland, atlantis and largecities. A few of them are described in the source like this:

• nodamage is like default without explosion damage.
• rarecities makes cities rare, and onlycities makes the entire world a city.
• tallbuildings builds very tall buildings and is marked performance heavy.
• safe has no spawners and no loot, but keeps the lighting.

Custom profiles are JSON files in config/lostcities/profiles. The file name without .json is the profile name, and the built-in profiles are written to that folder at startup as examples.

Common problems

• There are no cities. selectedProfile is probably empty, or the world was made before you set it. Set it and use a new world folder.
• The server lags while exploring. The tallbuildings profile is marked performance heavy, so try another one. Why is my server lagging? shows how to read the graphs.
• Something breaks at start. Fix Minecraft mod and plugin load errors walks through reading the log.