When standing in some areas I start taking damage, running a little bit away and the damage stops.  Returning to the same area and damage starts again.  This time I was in a cave near the surface taking damage and made it to the surface, as I was running away I thought I heard a skeleton bow "twang".  The damage speed and level seems more consistent with being attacked by a spider under the ground beneath me.  There were spiders in the tunnel below me at the time.