Hidden bonuses: 5
Level walkthrough: Exit the tunnel. Turn around quickly and use your bionic arm to get to the top of the concrete structure. The first bonus item can be found in the center of the platform. Head down and deal with a small group of enemy soldiers. Enter the tunnel once they're gone.
You'll encounter a new group of enemy troops when you reach a larger cave. Use your hook and some of your weapons to eliminate closer targets. As for more distant threats, consider throwing objects at enemy soldiers. Keep heading towards the relay station. I would recommend that you remain on the main path and on the ground instead of using your bionic arm.
You'll have a chance to collect the second bonus item along the way which will be hidden to your right. Make a jump towards a red platform seen in the distance and get rid of a new group of enemy soldiers. You should now consider climbing to an upper ledge to take the third bonus item. Head back down and use the relay station to disable the minefield.
Watch out, because an enemy Polycraft will arrive here shortly. You can destroy the flying machine using the special adrenaline move you've unlocked not along go. Aside from the Polycraft you'll also have to deal with several paratroopers.
Start moving towards the minefield. Look around to find the fourth bonus item and the fifth bonus item. Reaching the first object is going to be a little tricky, because you'll have to a make perfect jump using one of the mines. Getting the other item will only require you to lower yourself while being attached to the closest mine. Use other mines to exit this level.