Worldbuilding Challenge Day 21 - Underground Locations

What notable underground locations exist in your world?

They call it a mine, a MINE!

The Iron City of Brava has a large number of mountain ranges, within which are numerous mines that tunnel deep for ores and other precious resources. While there is an official mining guild that conducts routine operations, the mines are open to anyone willing to lay claim to potentially rich veins of Iron. One of the most profitable, and most dangerous, is the Blackheart mine. 

The mine has been the cause of several 'accidents' over the years, all of which the mining guild blames on faulty equipment or inexperienced miners. The miners who have survived it's black depths speak of a horrible creature, something awoken by the The Ending from long year of slumber. Whether or not you believe the stories, mining parties still disappear in the Blackheart mines, usually right after finding a new profitable vein of Iron.