Currently, we have a marvelous system for placing above-ground rivers. You can find the source in adventure mode, and trace it all the way down to the ocean. However, with underground rivers, instead all you get is this:
#########
###777###
##77777##
###777###
###777###
##777###
##443###
## ###
## ####
########
starting and ending abruptly. So, the natural conclusion is this: why not run underground rivers like the above ground rivers?
If the rivers are run like this, it solves several problems; one of the most important is the lack of the massive fish suicide when walking or departing onto an area with a underground river chasm. Not only does this prevent the annoying message spam, it also prevents the adventurer from finding underground rivers this way.
Another cool thing would be if the two types of rivers were combined. Imagine walking around, following a river, and seeing this:
, 777.,
777;
,777
^777^,
#####^
The river disappears right before your eyes. Puzzled, you jump in, and find yourself zooming underground, Tower-Caps and Plump Helmets in the misty dark, illuminated by some phosphorescent mushrooms and the occasional light bug. Eventually, your underwater adventure comes to an end as you appear back on the surface.
One problem that occurs with this is keeping underground fish, well, underground. We don't suddenly want carp eating all the cave lobsters! However, this is a minor point, and one I'm sure could be avoided.
EDIT: This is linked to in the underground diversity thread, so I altered the post. The original post was the following:
All I saw that was similar to this form a search was bloat 352; I didn't see any forum topics.
I think it would be a lot more realistic and interesting is cave rivers were generated in a similar manner to aboveground rivers. I'm not sure how hard it would be to erode away at mountains internally as opposed to externally, but I think it would add a lot to the game.
In addition, cave rivers really only make sense in limestone, so if they would be limited to limestone only, that would be cool as well. Then if the river tried to run down a mountain and run into a harder rock, they could run aboveground.