Essentially, when working with the destruction gadget, especially underground, there's the potential of running into either an open cave or a water reservoir, both being equally annoying, so here's a potential fix:

• Have the destruction gadget check for empty slots along the side walls (including top and bottom).
• Fill in those air blocks with some preconfigured block (kind of like the exchanging and building gadgets work)
  An alternative would be to replace anything (air or not) on the outside walls with a preconfigured block. This would be handy if you're building a basement or extension to one.

I haven't looked at the inner workings of the destruction gadget yet, but for the wall-filling what would be cool is if the gadget has a buffer of sorts of what it's destroyed, and uses blocks from said buffer to fill in the air blocks. I'm imagining a configable whitelist type of system for that.

All this could be wrapped in a button on the destruction gadget interface called something like "build walls", although thinking about it, this sounds more like a separate gadget than anything...

Thoughts?