'The rock sounds hollow, you will need to tunnel to proceed' while surface mining when no mine below.

[Arindor 27]

I have had this incorrect error present itself several times while surface mining.  I believe what has happened is I am surface mining at a location where at one time, long ago, a cave existed near the surface.  It has since collapsed and there is no longer any open cave tiles below where I am mining.  However, the server still "thinks" there is a cave there and will not allow the surface to be mined below the old cave height.

 

Originally I thought perhaps there was an unconnected void below me.  So I mined into the tile from below figuring to Strongwall it if I found a void.  If there had been any void, I would have mined into it or would have received another error while mining into it from below.  But there was no void.  However, mining into the tile, even without closing it with Strongwall, seems to reset the ceiling height and I was able to resume surface mining.  Of course, you need to make sure the ceiling of the new cave is lower than the height you want to mine the surface down to.

 

This has now happened four times, and in each case I was able to work around the issue by mining into the tile from below.

 

The "fix" for the server seems to be to reset/remove the ceiling height below when a collapse happens (when all tiles surrounding a corner are collapsed).

 

EDIT:  Associating support ticket #72787 with this bug.

Edited March 9, 2020 by Arindor

[Nicrolis 980]

This seems to be similar to my issue a few years back. I was mining at water level and ran into the messages for open cave tiles on the corners of where I was mining. When I tried to reroute to open those tiles from head on it said they were too far above(crack in the floor message). I found there were no open tiles in that spot at all. GMs came and looked and could not mine the tiles either.

 

Seems that every now and then tiles collapse and "remember" where the ceiling and floor levels were. this causes it to act like an open tile even though it is not.