I watched the video that wave form posted, and used that idea, of just using a shader on a block, put the block on the terrain, the terrain becomes transparent looking through the block, then run a trigger as you aproach, setting the player on a different layer, that doesnt collide with the terrain layer, then reseting the players layer as they exit the trigger. Gives a perfect illusion of a hole that is functional. Except, using an older unity terrain shader to do it, it only suppirts 4 textures on the terrain.... which i can work around, but what a pain in the a$$.