The Lower Mines are gotten to by going down the elevator from above in The Lower Dorn, which takes you down into a very large chamber full of crystals and of course, Gnomish Slaves who are working as hard as they can to meet their deadlines. It is here that you will meet a talkative Gnome named Tarnelm who will greet you as you come off the elevator and of course, he's a little skeptical of your party, but once you assure him you're not working for anyone and are there to help them, he will ask something of you. That will be to get some FOOD for himself and the other working slaves for they have not been fed for some days and are very hungry, telling you that food can be found in the Palace of Durdel Anatha. As well as the FOOD, Tarnelm wants you to kill all the Fire Salamanders above and when you've done both of these, return and speak with him so he can give you more information and another quest to complete.
Seems like there are Umber Hulks in the area searching for the hidden Gnomish Outpost and Tarnelm fears that sooner or later these search parties will definitely break through the earth and into the Gnomish Outpost and kill the remaining Gnomes who have not been made slaves. Tarnelm asks for you to search the Chamber for these Umber Hulks and take note of the walls for any cracks where a possible Umber Hulk has been.
If you haven't taken note of the Rhinoceros Beetles and Umber Hulks in the area, you will want to rid these foul beasts before you start your search for the burrowing Umber Hulks and the Gnomish Outpost. Deal with this threat now because you certainly don't want those Rhinoceros Beetles attacking you when you come across the burrowing Umber Hulks as they burst out from the very chamber wall.
Now if you're having trouble finding where these Umber Hulks burst out from, check the Lower Dorn - Lower Mines Enlarged Map for more details as to where you should take your party to find these burrowing Umber Hulks. There are nine Umber Hulks that you will have to deal with and could cause you some trouble if all your fighters end up getting confused and attack other party members. Regardless, you will have to defeat them all, so do your best and then return to speak with Tarnelm.
When you're ready, return to where the Umber Hulks burst out and let's go visit this hidden Gnomish Outpost Tarnelm was speaking of.