This is the best place to mine Mithril that I have found. I used
to keep it secret from the rest of the world. It's very secluded and
hidden deep in a cave on the Aisle of Dread. The cave is infested with Nagas that have probably the fastest respawn rate I have seen anywhere in
the game. If you can handle taking on lv 45ish casters and warriors,
you will do just fine and get a TON of Mithril. I actually used to mine
here at lv 60 all the time and made more money than I could mining Thorium. It's also rich in Truesilver. I used to go in for about an
hour and come out with about 50-100 mithril, 10-20 truesilver, and a
handful of star rubies. Here's a map of its location.
Head to Feralas and fly to Feathermoon Stronghold. If you don't have the flight path, it's a long hike to the island in the southwestern corner of Feralas. Once you get there, go straight south to the coast and follow it south. Go along the southern coast to the west until you see some ruins. Then cross the channel to the south until you wind up on the Aisle of Dread. Follow its shoreline until you find some more ruins infested with Nagas. This is where you want to be. Battle your way past them and you will get to a cave. This cave is full of Mithril deposits.
The biggest problem is obviously the lv 45ish Nagas. They respawn very fast and are all over the cave. If you don't keep moving, you can get respawns behind you and end up surrounded. Pull them back and keep a good sized window of cleared space around you. There is a silver elite that spawns occasionally in there. She gives pretty good green drops too. You will get constant supplies of Clams so be sure to loot them for the pearls they contain. The nagas are excellent exp. I actually grinded in there from about lv 43 until 50 or so when they became gray to me. If you're higher level and can go past without really being worried about nagas, you can log off in the cave, log in, mine the Mithril, then log back out. I used to camp my 60 hunter there for weeks at a time while I played other chars. I just came back out to empty bags and went back to it. I made hundreds of gold that way.