Is making maps of vanished parts of the world. I get to "see" underground in 2D echo-sounding profiles and even in 3D. I can map drowned islands, lost rivers, infilled seas and fossil shorelines.

It is so wonderful being able to recreate, albeit on paper, the visions of a lost world - just one timeslice of billions that our Earth has endured. This planet is so dynamic that it beggars belief.

That's one of the reasons why I get so cross when people call Mars "Earthlike". They don't even begin to understand what Earth is like - they have a primitive and simplistic view of the Earth as a passive lump of rock that just accumulates rivers and stuff through time. In fact, the Earth is constantly renewing itself and redirecting everything. The battles between growing mountain chains and established rivers are epics of biblical proportion, spanning hundreds of millions of years, and ending up shaping the world by building wholly new lands.

Mars, on the other hand, is an accumulated stack of impact craters and lava. Slice into the ground anywhere and you see the profiles of impact craters outcropping in the walls. The processes that on Earth would destroy craters in a few millenia are so inactive on Mars as to astound the careless thinker.

But to return to the topic of palaeogeography - we mostly do this on paper (or in PowerPoint, these days!). The best visionaries can create gorgeous 3-D block diagrams, but we don't yet have software to automatically generate fractal mountain ranges and place tributary stream systems to infill the coarse data we have.

I think it will come, though, in another decade or two. (When I was at University, I dreamed of such software, too, but it was the sort of thing that was way ahead of its time. Now I can see the technology catching-up with the need. Basically, you need the sort of technology that makes digital film animation, on your desktop. Moore's law will bring us that in a decade and the geological software people will catch up in another...)

(And you will even be able to zoom-in and see little dinosurs plodding through the landscape!)