I'm assuming this is a question in regards to Game Guru Classic.
The Lua command is:
HideTerrain()
If you dont want to write the script yourself, you can use the script "goindoors.lua" and put in the a Trigger Zone near the player start marker. This will hide the terrain and water.
If your issues are related to FPS drop, you should consider making the terrain completely flat. You can do this by flattening it within the editor. Or using the super flat option in setup.ini. You can also reduce the terrain quality by reducing its shader in the TAB TAB menu and set to Lowest. You can also change the Terrain Size to 1, so it doesn't draw it in the distance. If you are making an indoor level, you should also reduce the Camera Size to be as close as possible without walls disappearing within few. You'll gain FPS with this process.
Additionally, if your map is an indoor map, make sure to turn off all vegetation sliders in the TAB TAB menu settings and reduce its shader quality to "Lowest".
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