What causes the circles on the greens at Carnoustie?

The Game Room
1

Sprinkler system?

2

Probably. Or it could be the current drought revealing the remnants of ancient settlements. It’s more likely to be the sprinklers though.

3

I think it’s Fairy Ring: https://en.wikipedia.org/wiki/Fairy_ring

4

I’m pretty sure they are discolouration caused by a fungal growth.
A fairly common phenomenon, I’ve seen loads on playing pitches and golf courses.