Once through there you can either walk straight ahead or turn right through another door. You want to walk straight ahead to the area called the Dark Iron Highway. In this area is where you are going to make tons of gold mining ore. There are numerous dark iron spawn points in this area and on each run you should be able to get to one or two of them. Simply mine the ore, leave and reset the instance to do it all over again. You will need a couple of people at least for this or if you are a rogue or druid you can stealth in. Also, hunters can run in and feign death to escape the mobs around the ore.
Gold farming in the WoW blog  is a controversial topic as recently stated by the NY Times. It is only controversial because of rampant abuse of the system. There are some who set up accounts just for the purpose of farming gold and illegally selling it to other players. This is strictly prohibited by Blizzard's WoW terms of use. However, it is not against the terms of use to farm gold for yourself, your guilds, your friends, or any of your own personal accounts. The following Gold farming strategy has been proven to work in the game. A word of warning about the mobs in that area-there are bloodhounds in this area that can see through stealth from very far away for their level. Also, they hit very hard but fortunately don't have much health. You may have to fight a few of these to get to the ore, but they can easily be soloed if you are careful.