An abandoned heavy fishing net.
Not something I want to repeat, quite scary actually!. I was a real struggle to get the anchor up, but there was enough "give" to stop the anchor rope breaking. When we realised what it was the concern was not to get the boat tangled in it. It was in quite sloppy conditions, an afternoon NE had kicked in and we were heading in.
We came close to cutting the anchor rope a few times but managed to finally free the anchor. We would have like to have taken the net in with us for disposal but it was just too dangerous.
As I said, not something I would like to repeat.
Cheers
Dave