The MMTerrain Engine is designed to run virtually unlimited tiled terrain sets with a minimum of setup in Unity. This is mostly applicable for simulation and GIS applications that need to quickly render large terrains with an immediate level of detail as needed.
Example videos and docs can be found here:
http://www.freeterrainviewer.com/MMTerrainForUnity.html
Provided that resource management and load-balancing are implemented on the client side (factoring in Unity pipeline limitations), it is also possible to use tiled data for flight simulation or constant-FPS applications that require smooth performance. A procedural game-style mode is also available as an optional bonus for artificial environments, though the intended purpose is for large geographic data that cannot be otherwise rendered in Unity at all.
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