The Astrub Underground is a mine. The entrance is at [6, -19] with the Sick Grossewer Milirats.
It's where most first missions like the Xelors and the Ecaflips end. Most Sick Grossewer Milirats are lvl 10 or 13; the lvl 13 tend to have Tofu (lvl 2 to 4 usually) with them. Not really meant for low level players as they can attack twice in one turn, casting punch.
There is a mission in there as well which ends by killing a lvl 30 Sick Grossewer Rat. If you really want to fight it, do it with higher level people; if you are already a high enough level you can tackle it for some decent experience.
Astrub Underground is the primary place to level new characters, it features spawns such as Tofu, Arachnee, and Sick Grossewer Milirats.(this is no longer true due to the recent addition of incarnam)
This Underground (p2p area) also has an exit to the castle of Amakna. However there are full of high level rats.