Tony Hawk's Underground introduces a new system: walking. You can get off your skateboard to run or walk around the level. If there's a high tower you want to jump off of, get off your board, climb to the tower (by climbing a ladder or shuffling across a wire), get back on your board, and jump down. One of the new tricks associated with this is the Acid Drop, which can launch you off an opposing pipe, allowing you to pull off 1080's and up.