In the Woods level, you will reach a clearing where you will then be attacked by Deadite Dogs. After you defeat them, you will see a bridge that is up on a ledge. To right of it is the Hidden Extra. You must progress through the level until you reach an area where you must ride a Deadite with Sam. After you kill it, you will encounter the Harpoon Gun in a mining area. Fight your way back towards the clearing, and before you reach it, there will be two small cliffs you can climb up. Once up on the ledge, target the hole at the top of the bridge with the Harpoon Gun and it will bring the bridge down. Go across to collect the Hidden Extra.