(spoilers) What does she see...

Right after she kills the firemen guy with his axe? She's in the tunnels hholding on to the walls to get out, and she starts screaming. I saw the baby, which I understand is insinuating that the story about the dead babies and the nuns going into the tunnels to have sex is true..
But then the camera keeps going and it shows something with an eye that looks like an animal or something.

The camera stayed on the eye more than a second while the music was climaxing so it's definitely something they wanted us to see, but I don't get what it was or what it meant.

It's possible I missed something earlier that would explain it. Did they say something about testing being done down in the tunnels or something? That's all I could think of!