I just pushed out 12 new combat battlemaps to all subscribers and license holders of Fantasy Grounds. They can be found in the FG Battlemaps module that will download when you run an update. These maps include 1-3 separate tunnels that can be used with masking to represent tunnels in the Underdark. We will probably include these directly in our version of the Out of the Abyss adventure module when it launches on September 15th as well. Here are three of the maps below.
There are versions with a 5' square grid and ungridded versions available as well. The scale is 50 pixels per square.
Sample