Turf Blister

That is mother natures very own pus boil.

Is it me, or is that "blister" shaped like a heart?

It happened in a neighbor's yard when I was little. He had new sod installed a few years earlier and the grass grew together and into the underlying soil. However, in that area, the ground was extremely rocky and hard, so the result was that the roots stopped at the point where the turned soil met the native dirt a few inches below. When a pipe broke under his lawn while he was on vacation, the entire thing filled up with essentially loose mud for over a week before it met the sidewalk.

BWOP! RUSH! Hundreds, if not thousands, of gallons of muddy water shot out and filled the street.

I guess a tough layer of grass could hold together above a leaking pipe. I'm surprised there was no splashing or tearing though.

This happened in a park in the UK when I was but a lad. It was a combination of very wet conditions, and a burst gas-main.

we (being teenage boys) dug holes and lit the escaping gas!

It looked great at night!

Fortunately the gas was bubbling up through the sodden mud underneath, or I guess I might not be here to tell the tale!

I have actually seen this happen in RL before, in my case it was a broken water pipe under the ground.

Water pocket. I've seen them before, but none so durable or large.