1 - To the right of the Grenade Launcher there is a cracked wall. To the top left of that wall there is a large metal tube. Shoot a grenade into the tube to activate the secret.

2 - To the left of the large gate there is a pool with 4 electricity pickups around the center. There are 4 torches located around the pool that are not lit. Using the Burner, you can light all 4 torches to activate the secret.

3 - To the left of the Crossbow, tucked in a corner, there is a box with sparks coming out of it. Shoot it with the Laser Gun to activate the secret.

4 - On the opposite corner to secret 3, on the other side of the pool that is close to the Crossbow, there are 12 axes lined up and a bullseye. Use the Crossbow to line the shot such that the arrow goes through all 12 axes and hits the bullseye. This will activate the secret.