Area 5100 is the 31st level in Serious Sam 2.
With the Medallion completed, the Alliance can finally start their assault on Sirius, Mental's current residence. Sam is warped to Kronor, Sirius' moon, in order to rescue some the Alliance prisoners on the planet, secure a location on Kronor for the Alliance to use as a starting point on their attack on Sirius, and use Kronor's Laser Cannon to damage the shield protecting Sirius long enough for the Alliance to enter and start a first strike.
Sam teleports into a hanger. As soon as he comes out, an alarm is sound, which summons many enemies to his location. As expected, Sam manages to clear them out thanks to the weaponry and Plasma Turrets he finds. With the initial attack force dead, Sam rescues some Alliance prisoners. He then travels to another prison block and wipes out the enemy forces guarding it, then rescues the prisoners in that cell block. With most of the required prisoners saved, Sam enters an enemy-infested tunnel that connects the prison blocks to what appears to be an outdoor warehouse.
1. As soon as you exit the hangar, go left until you hit a wall. When you've reached the wall, turn left again, and you'll se a part of the hangar that you can climb up. Go up it, and you'll find an SBC Cannon and three Cannonball pickups. Grab the cannon to register the secret.
2. Go back to the hangar exit and continue walking until you see an acid pool with with shark fins in it on your left. Now, go to the left side of the acid pit and you'll see two pipes with wheels on them. Use each wheel. This will drain the acid pool. When it's drained, jump into the pool and pick up the Super Health in it. This will register the secret.
3. On the left-most wall just past the acid pool is a gate with a tower next to it. If you get close to the gate, it will close and you'll see a Gnaar in the tower. Kill it in order to open the gates. Go into the area the gates/Gnaar were protecting, and you'll find a CH-181 Hover Bike. Hop in it, and you'll register a secret. However, two T-Mechs will spawn in the large area before the gate and three more T-Mechs will appear at the top of the slope past the MK 4 Grenade Launcher as soon as you hop into it, so be ready for a fight.