In what can be classified as a fourth secret, near
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 ammunition
at the bottom. You can continue going around and head to the mummy
statue the back way, and finish the level without the thing dropping
down the hole.