An old tyre

Now this is a bit of a mystery?! I went walking in the fields today and look what I found ......................... an old tyre. I thought it might be something that I could play with but it was so heavy I couldn't even budge it, let alone roll it along the ground so I could chase it.

It got me thinking - how had it got there? We weren't near any roads and I bet a tyre couldn't fall off a tractor without the farmer noticing????

....................... So it's still a mystery to me!

Any suggestions?????????????