In what can be classified as a fourth secret, toward the END of the level when you emerge out of the water outside with the fences, after taking out the nearby foes, walk right next to a pit of spikes to the corner of the fence. Line up with the corner, then do a roll and you should END up on the other side of the fence. You most likely were not meant to be going out there. If you go too far toward the starfield, you will suddenly drop dead. As to what is actually out there, there are a few more enemies to deal with, and another pond, which actually has more ammo at the bottom. You can continue going around and head to the mummy statue the back way, and complete the level without the thing dropping down the hole.