Mine hole over cave

[Elchupac]

I understand that you can enter a cave by building a mine hole on top. What happens to the stuff right under ? Let's say there is a house underneath. Or even a claim post. Are they destroyed ?

[MagicManICT]

If there is an object underneath where the mine would open up, you get the message "You cannot build that here" or something like it just like if you tried to build over a chest on the same level.