Many of you reading this will no doubt be all too familiar with Sonic Adventure (it was one of the first games many early adopters experienced - including me), but as this video shows there are still things to learn. Prior to watching it, I had never heard of the Sonic Adventure AutoDemo mentioned in reference to the Casino 'cow girl' billboard, and a quick Google search later opened up a whole new world of wonder to me.
The AutoDemo was an early rolling demo of Sonic Adventure (dating from October 1998) that was used to show off the game's engine - it basically shows a load of cut scenes using in-game assets. Turns out a group of Sonic Retro forum members (headed by a user named 'Orengefox') managed to hack the game though, and released a whole load of previously unseen development assets and pre-alpha levels. It's a fascinating story and you can read all about it here and here. While I was aware of the recent Windy Valley discovery, I was previously ignorant to how deep this particular rabbit (echidna?) hole went...