Terrain has random dark shadows on it

Hello, first post and total newbie here, so this is surely obvious, but I can’t find any info online to tell me what’s going on.

I’ve been playing around with making a terrain and along the way have probably clicked something I shouldn’t have clicked. The terrain now has several dark/shadow blobs, which are apparent when viewing plainly in the editor, and are very apparent when testing because everything is vastly darker when you walk into those areas.

I’ve tried deleting/replacing all the light sources that exist. Nothing seems to work. And I can’t find any objects in the scene that are designed to cast shadows like that. Really lost here…

See below images.