Therion is the next step in cave surveying software. It works under Windows (9X-2000-XP), *nix and MacOS X. It's GPL-licensed.
Usually, cave surveying software works with survey data only (from, to, lenght, compass, clino, etc ). From these data, a centerline is calculated, then this centerline is exported to drawing software like Illustrator, where walls and details are added.
Therion is able to work with both data and draws. There's no need to export centerline data to drawing software. You can draw a line representing a pit, then set this line type to “pit” and Therion will draw a pit line as accepted by UIS or other caving associations. So you don't need to break Illustrator's restrictive copyright to achieve this.
Therion is able to distort your drawings to adapt them to changes in your centerline. If your centerline data changes, usually you need to redraw walls, etc. This is not necessary with Therion. Their algorithms take care of things like underlying passages, etc. So your maps are always up-to-date. You don't need to redraw after loop closure, blunder fix or set symbol change.
Therion is able to draw single-page maps. Maximum page size is 5 x 5 meters. It is able to draw a cave atlas too, with overlapping borders and navigation icon on each page.
Therion is able to build 3D models without LRUD data. They are calculated from walls you draw, so Therion's 3D models look usually better than “square” 3D models made by other cave surveying software. Surface data can be included.
Finally, Therion is able to export your centerline data in SQL format, ready to be loaded into a database system, so you can do exquisite searchs on these data, or use it on GIS software.
This guide assumes that you aren't a geek-caver, but a plain caver with little or no IT knowledge. If you are a geek-caver, or need to use advanced features, you should read thbook.pdf. It is in your therion/thbook directory.
There is another non-geek guide, Wookey's one, called “Therion - State of the Art Cave-Drawing Software”, available in http://therion.speleo.sk/download.php.