Mega Tiles are just what the name implies: really big geomorphs compatible with Dungeons in Blue tile sets. Each Mega Tile is a single geomorph covering four times the area of a standard-sized tile. These larger tiles allow bigger caves and chambers to be depicted, and also serve to break up the gridded feeling geomorph-based dungeons can sometimes have.
Mega Tile Thirty One shows a complex of rooms and halls with many trees (stone? magical?) scattered throughout.