What does the Darkness tile underground do?

I was exploring underground and mining out some rock to get to the big black splotch on my map. so I got to it but nothing happened, can’t walk onto the darkness tile either, is it some kinda bottomless pit?

Also

I LOVE this game, its awesome. Good job!
I only encounter one bug, if I play for awhile, I start getting “an error has occured” message whenever I try do or move something, but it goes back to normal if I save & quit and then reload the game.

And! is there more alchemy recipes besides the ones in the wiki?