>Of course, the shape alone tells you which direction to point it (for at night).
>
>What I'd like to know is why they call it a "mine".

Has to do something with mining (ore), because they use mines (expl.) to make their mines (tunnels) go deeper, i.e. to break the stone. Actually, in several non-English languages the there's one verb for mining (ore) and another for mining (setting mines (expl) to explode to dig a mine (ore)). In Serbian (et al), "rudar" is a miner (ore), because "ruda" means ore, while "miner" means "miner" (expl), because "mina" means mine (expl).

Clear?