How to build a large terrain of 240km*240Km with unreal engine 4?

We want to build a large terrain of 240km*240km with UE4 Composition, and we use the tool “World Machine” to build tiles of landscape.But When we import tiles to UE4,the length and width is less than 240km, the height is below zero.
Whether has anyone met the problem?

just curious what kind of project are you working on where you need a map that large