This can be done in Chapter 3. The Alpha spot is at 36,17.
Go to 36,18 first and stand on the small hill (next to Malie if he’s there). Turn into a ball (X) and use your speed boost (R1). Grab the ledge on the left and go all the way to the top of the room. Turn into a ball again and use your speed boost. You'll want to aim for the big gap on the left. There’s a ledge on the left, with a hidden path. Follow it to the next room (36,17). You'll see the Alpha icon right in front of you. You can't reach it from here though, you'll need to walk forward, but be careful, you'll drop down. Quickly turn into a ball again and use your speed boost once more to reach the Alpha spot.
Credit to: http://www.ps3trophies.org/game/knytt-underground/guide/
Credit to: Mr Vuule for the video