You can find this area in the Toxic Refinery (Episode 1, Level 3). Complete the first half of the level (before you enter the blue door), then go back to the starting point. Look inside the pool of radioactive waste and you should see a platform (if you flipped all the switches in the first half). Walk across the platform towards the wall it leads to. The wall will open and you will have to defeat a series of enemies. Explore the secret area more and you should find an exit door. Enter it and flip the switch (the stats will not appear). You will now be an area called the Military Base. Note: This area can be very difficult.