Recently a group from the University of Washington in Saint Louis scanned a 4 km section of Mammoth Cave in Kentucky. An interesting article about a project can be found here:
http://www.sparllc.com/archiveviewer.php?vol=08&num=14&file=vol08no14-04
The group was able to merge over 130 separate LIDAR scans comprising over 18 million data points to visualize 4 km of the cave. On a whim, we reached out to the principle investigator and he shared a version of his data set that comprises around half a million vertices.
Visit the cave by searching for it within TerraViz.
