While we wait for Descent:Underground to come out, it's still possible to play Descent 1 and 2. Here's how to do it:
1) Get a copy of Descent from your old floppy disks/CD/Hard Drive where you used to have Descent.
2) Install the latest retro mod for rebirth:
-- On Windows, run the Rebirth Installer . On the screen that says "Select components", make sure to include Retro Mod and the mission packs (all under "downloadable content" -- these are separate for D1 and D2.) To play, run d1x-rebirth-retro.exe or d2x-rebirth-retro.exe (there should be desktop shortcuts; if not you can run it directly from your D1 or D2 folder.)
-- On Mac, download Retro for D1 and Retro for D2 . Extract these to your Descent folder (whichever folder has the Descent.hog and Descent.pig files.) To play, run the Dxx_Retro_1.3 app in your Descent folder.
-- On Linux, compile from the source on github (you'll want to clone the repository and run scons on it, targeting the d1 retro 1.3 and d2 retro 1.3a builds. I don't have any further details on how that's done.)
3) If you didn't already get the mission packs from the installer in step 2, download them packs for whichever game modes you want to play: D1 Rangers Anarchy, D1 Rangers Coop, D1 DCL 1v1, D2 Rangers Anarchy, D2 Rangers Coop, D2 DCL 1v1. Extract the mission packs for whatever game mode you want to play to the "missions" folder inside of your Descent 1 or Descent 2 folder.
4) Join the same voice server as the people you want to play with. For Descent Rangers, that's mumble on mumble.descentrangers.com port 31393. For the unofficial Descendent Studios Teamspeak, it's at descendent.ts.nfoservers.com .
5) To host a multiplayer game, you need to have port 42424 (UDP) on your router forwarding to your computer. (You do not need to do anything special to join, only to host.) Pretty much every router has a way to do this, but they're all different -- usually in a section called "networking" there's a subsection called "port forwarding" or "virtual server" that allows you to do this. You may also want to set up a static IP for your computer when you do this, so that your port forwarding rules will keep working even after a power outage.