How to add a collider to an uneven surface(Ground)

It’s not a Terrain rather a spongebob map I picked up and the problem is that I don’t know what Collider to add to this ground.

Adding a box collider is obviously stupid and Mesh Colliders also don’t work.

I’m super confused what to do with this i don’t really find any yt videos so fast answers would be appreciated.

Any ideas fellas?

can you explain why mesh colliders don’t work ? mesh collider should work here and that is the way to go…