Those are not caves. They are.. lies. Lies upon lies, on a map.
Try to use more polys in your next map. Choose a theme. Draw the layout. Place the polys, and try to smooth them out. Once that's done, add color and shading, and place some sceneries to help liven it up (but don't go overboard).
Shading:
There are a few ways to shade, mainly just stick to the lighting in your map. If you have an enclosed map, such as yours, use sceneries to portray light (things such as torches, lights, etc.) and shade accordingly.
If you are unaware,
Anna's PolyWorks is an excellent resource for new mappers, and may help you with all I've said and more.
Good luck.