Tunneling on a shallow [[slope]] is dangerous since you are basically mining straight down and may dig yourself into a steep slope or a pit. A slope of 10+ is probably safe (estimate). The good news is that a poor tunnel slopes can be fixed. The bad news is that it is a lot of work!
Tunnels can not be opened if the lowest corner is more than 4 dirts below [[Water#Water level|water level]].
=== Surface mining ===