something yellow

My Rhubarb continues to thrive in the fall weather and rain.

Next to it, a bright yellow patch. I thought from the window it was a toy my kid left out there, so went out to retrieve before the rain (we've had a few blankets and dolls left out in the rain turn moldy and have to get discarded lately). It wasn't a toy. It was a big fungus.

Here's the other side.

It was growing right where the fourth, dead rhubarb plant used to be. It's not the only one, though. There are several others in the weedy part of the back lawn, scattered about ten feet apart. I wonder if it's all part of the same underground system? and what kind of fungus it is...