Help?

of the islands. There's just a bit of land left that it won't let me break. Does anyone know how to fix this?

It could be that something is too close to it. Or, and this is something I have done, you used the wrong tool to try and remove it.

You will probably have to use the water tool to make a land bridge back to them. Then you’ll have to destroy them blocked by block, as you make your way back to the edge of the lake.