Wood constructions can also float in the air without a connection (depending on how high you built them and what size parts you used).

The silo I built at this base was for decoration until I could build a windmill. I used the shortest wall pieces and I removed the entire lower level of walls and it didn't start collapsing until I removed the 2nd lowest level of walls.
https://steamcommunity.com/sharedfiles/filedetails/?id=2853688746

Ore nodes will float in the air if you dig out around them until the soil is too far away or until you strike the node in the exact center (assuming you have cleared away all the soil) at which time it will shatter into a shower of ore and stone.
https://steamcommunity.com/sharedfiles/filedetails/?id=2856429325
If you dig around large boulders they will sink (and if you fill in terrain around them again they will pop back up. You can't bury them).

Actual terrain doesn't move when you dig on it or under it so you don't need timbers to support a tunnel and its possible to dig so that you have a patch of dirt floating in the air.

The large flat stones in the plains and black forest that look like copper deposits behave like terrain.