they have lots of other sense to rely on (they have chemosensibility, smell and taste if you will). and they can feel a pressure differential. the soil beneath the topsoil layer isn't where they wanna be probably (because its not very nutritious since it has a very low organic material content). it may be instinctive for them to get outta the dirt when its all waterlogged. but i'm sure they have a way to sense which way is up. if they didn't they'd get lost all the time. for some reason certain species (like "nightcrawlers") like to come up outta the dirt in the late night early morning anyway (which is why birds go worm hunting early in the morning). i think they do it to clear their tunnels of the "castings".