[Tristan Lall]

If you guys are really serious about this--and that means that you want to mess with the engine, rather than just making mods (which is fine too!)--Descent and Quake II (or was it Quake I?) were open-sourced, if I remember correctly. Descent is missing some proprietary sound driver code, but other than that, is complete (and runs under DOS or an emulator). Quake, I'm not sure.

But really, a modern engine like Unreal, with a well-developed modding community is probably a better choice, if it can be used without cost. You will have to consider people losing interest in the project (it always happens with mods), and will have to evaluate the technical aspects of what you want to do versus not only your abilities, but also the capabilities of the modding layer of the engine. (What do I mean? You can't mod Warcraft II into Splinter Cell--though for your purposes, a 3-D shooter engine should serve nicely).

Consider which engines or games can be used to approximate real-world physics. Unreal Tournament isn't so great out of the box, but Raven Shield (an Unreal-based game) has rather good physics.

Also consider that these open-source engines, which may not have actually been used in any products on the market, are (by and large) unsupported and unproven. Will they work? Hopefully/probably. Does Unreal (for example) work? Yes. (And can you afford to buy the source code? No. Would they even sell it to you? Probably not.)

But take a good hard look at the potential for people to back out of the project. If the only person who can handle the advanced C++ and DirectX (or whatever you're coding for) needed for a full-fledged game engine quits out of frustration (or petty bickering, or illness, or school, or life in general), what happens? You need to make sure that the project depends on no one person, or that every person will absolutely refuse to quit (which is unlikely, at best). Wouldn't it be a safer bet to mod a game, where there is comparatively little skill required, and where tasks can be assigned to just about anyone? (You probably shouldn't assume that someone who just completed AP Comp. Sci. is able to write--or even read--code to support DirectX, but maybe they'll know enough to adjust parameters in data files, or learn other skills from modding tutorials.)

In short, don't get in over your heads, and you'll be fine. (And if you still want to go ahead with tweaking the game engine itself, just keep these observations in mind.)

P.S. The link to Crystal needs an extra "e" in "sourceforge"....