As the years have gone by, you have dug down and deep. This has
had its price. Arska reported that the ground has collapsed here
and there all its own.

So, the next time you build a tunnel, don't be amazed if you happen to
stumble into an abandoned tunnel deep down in the earth.
As always if somebody left anything behind, finders keepers.

Have fun.

Many many thanks to Ggr for his help and contribution.

~/A

PS. Effective the next time you create a new tunnel.