Originally posted by Lorska:

What exactly can't you take out?
The only terrain that's indestructible should be set pieces like rune stones, dungeons and the like.

It's just a chunk of the ground, everything else around it was able to be flattened to at least below sea level throughout the whole channel. I used a pickaxe and hoe for the rest of it and these two humps of land wont recess and it makes aquatic travel impossible.