Some other large-scale underdark regions are covered in Dungeoneers Survival Guide for 1e (maps with sparse fluff), The Night Below for 2e (immense campaign-in-a-box), and the Forgotten Realms Underdark hardback for 3e (crunch, fluff, maps, the works). These should provide plenty of material to work with, whatever edition or setting you play in, with varying amounts of work required. 3e's Dungeonscape is also good, if you are looking for general advice and 3e-specific crunch.
There was also an awesome, awesome thread here on ENWorld about games set in and around the Sunless Sea, but I lost the link and can't remember what it was called. It was fantastic, though, brimming with outrageous ideas - cities built on stalagtites, ships made from giant undead beetles, weird climactic effects etc etc. Anyone got a link to it?