Assuming there is a prize: my guess is an ancient rusted (pre lost arrow?) pin that someone reslung and then bashed farther in using another pin as a piston. (Veeery clever for that guy, veeery spooky for those ever after.)
OMG! 2008? Was this on the OW pitch? That thing is still there, same cord and all. I wondered if it was a hex that had been fished down through a tunnel above (can't see it in this photo).