Download Easy Open World Free Unity.
This tool allows you to get quickly started with a seamless open world game.
The world is split into smaller scenes that are automatically loaded around the player. The size of the world and tile size can easily be adjusted to your needs. One base scene is always open that can be used to hold larger elements like terrain. Terrain can also be split into different tiles (see preview video).
Custom editor UI is included to easily navigate the map and load/unload tiles.