What in the world is that kind of turf? The only grass I'm familiar with that -might- form a dense enough root mass to do that would be zoysia. And you wouldn't want to be walking around on that in bare feet.

*water *nature

Adding video to channels (Nature, Water) - requested by ant.

On the bubble or on the zoysia? Based on my limited reading zoysia is really soft.

Surely some kind of lining laid under the turf?

It's probably commercial turf not grown from scratch. To enable commercial turf to survive harvesting , transporation and installation, immediately after seeding a field they roll out a 20' wide biodegradeable plastic mesh which becomes incorporated into the sod as the grass grows. I've actually had a bubble like this beside my house, but much bigger, caused by a glitch in the irrigation system.

Moving this video to bobknight33's personal queue. It failed to receive enough votes to get sifted up to the front page within 2 days.

I had to think back a bit...it's been a few decades since I spent any time on a lawn of zoysia. But once I thought a bit more carefully, it does seem that zoysia changes it's texture with the season. Over the winter (and dry times) it gets tough and spiky, like walking on a bed of nails. But during the (limited) times when it's fresh and green, it is pretty soft. I think you can tell which season sticks in my memory.