Mysterious Stinky Thing in my Flower Bed

So, every year at about this time we have had Bad Drain smells in our garden. This has been going on for over ten years. I always spend a few days sniffing at drains and being mystified when our drains appear to in fact be fine.

A few times I've decided the stink was coming from the earth in our flower beds, but could never narrow it down to a specific location. I've wondered a few times if it was actually a Bad Drain smell that was heavier than air, sinking down to ground level and then drifting about aimlessly, hence hard to locate.

I've had this lingering paranoia that we have a broken sewer gradually seeping shit into our earth, but had no idea how to ascertain whether this was true or not, or what to do about it if it was, so shoved it to the back of my mind.

ANYWAY. Tonight I went in the garden and was hit by how strong it was. And also found the dog had got through the makeshift fence and into the bit she's not allowed in, where she had happily dug a new hole in a flowerbed. Got the dog back inside, and suddenly the stink was all over the house. Took me a while to realise that it was only in whatever room the dog was in. And her feet stank of it. So I checked out the newly-dug hole... yup, it stank of The Stink. Oh no, I thought. I was right. We have earth full of sewage. So I took the torch out, and found... really weird stuff in the earth, where the dog had dug a hole.

There was a load of crumbly whitish-coloured stuff, a bit like soggy chalk or crumbly cheese. It stank of The Stink. There was also a thing that looked rather like a potato chopped in half. You know the patterns you get in the cross-section of a potato, kind of radiating out from the centre? Kind of like that, and shrivelled at the edges, like a dried-out half-potato. But also slightly spongy, and grey. I didn't investigate very far, cos it was dark, and it stinks. Although you have to get your nose quite close to it to smell it. But sadly I got some of the crumbly stuff on my fingers and the smell won't wash off. But again I have to put my nose close to smell it. But anyway. There was a significant quantity of the weird crumbly stuff, like, more than a handful, and it looked like it might extend deeper. The half-potato thing looked like it might also be crumbly and like the other stuff if I was to break it apart.

So, could it be some kind of stinky fungus? Maybe it's been there for years and comes into some kind of Pong Season at this time of year? Any Smelly Fungus experts out there?

[update: see here]