The Sinnoh Underground is a huge network of tunnels.
It runs underneath the region of Sinnoh. Once you reach Eterna City and obtain the Explorer Kit item from the Underground Man in Eterna City, you can enter this huge area. He sets you different challenges to complete in the Underground. These range from digging for spheres to making and decorating your own Secret Base.
Spheres are the currency of the Underground. Spheres can buy you anything, from traps to treasure and even items to decorate your secret base. If you bury spheres in front of you and leave them there after a while they grow larger. If you bury a lot of spheres of the same colour in front of you and then dig it up a larger sphere is obtained. You can also find Fossils, which can be restored in Oreburgh City, Heart Scales, which are to give to the Move Tutor so he can teach your Pokémon other moves that it has learned by leveling up, and rarely Evolution Stones. Once in the game, a player may also obtain a random Plate by digging for spheres. On the floor, traps can be picked up. People can play together using DS wireless Communications in the Sinnoh Underground. They can steal flags from each other's secret bases to upgrade thier own. They can also interact and "chat".
This is an improvement on the Pokémon games. You can go down there anywhere you like (with a few exceptions, like the Distortion World and inside buildings).You can bring up treasure up from the underground and sometimes sell it.
The underground changed greatly in Pokémon Platinum. Capture the flag, traps and rooms were given a boost.
As of Platinum, you may use WiFi to connect with friends to do Co-Op exploration or capture the flag. The underground changed the Generation IV games. Other than the WiFi plaza, the underground is an extremely popular Nintendo WFC spot.