I think instinctual responses are the greatest challenge. Many things that move on the surface are tied to instinctual impulses below the surface.
Leviathan, the twisting fleeing serpent lives in the deep, and must be slain by the sword of His mouth.
In that day the Lord will punish Leviathan the fleeing serpent,
With His fierce and great and mighty sword,
Even Leviathan the twisted serpent;
And He will kill the dragon who lives in the sea.
In that day,
"A vineyard of wine, sing of it!
"I, the Lord, am its keeper;
I water it every moment.
So that no one will damage it,
I guard it night and day. (Is 27)