Here are some simple custom dungeon and mineshaft brushes for World Painter that I have created. About 2 hours was put into creating, converting and resizing from the original concept.

Note: These are best used as underground caverns and dungeons then these may help (using the add "custom cave/tunnels layer" tool). Just linkup the areas you wan with a normal brush afterwards (if that is the desired effect).

Or you can use some (high quality) brushes to create a new world using "import new world" and select "from height map". Just remember to choose an adequate size by increasing the scale by a few hundred percent. You may not want to do this to scale as that is thousands of meters (1 minecraft block =1metre).

Notes:
You can access and add to the brushes folder by going to:
C:\Users\(user name)\AppData\Roaming\WorldPainter\

There is also a brush folder button in World Painter under tools.

Enjoy,

Rogue