The Underground is one of the most ambitious features in Pokemon Diamond/Pearl. As a local-wireless multiplayer feature, it's... quirky, at best. Okay, we're lying. We think it's pretty boring and stupid as a multiplayer game. That said, it's an invaluable resource for Pokemon training, breeding, and trading. There are tons of valuable and useful items to be found in the Underground. Here's why you should spend at least an hour a week digging for treasures in the Underground.
Above: A Manaphy with Rain Dance is more effective with a Damp Rock. Get one in the Underground!
You need evolution stones to get some of the best Pokemon and to complete your Pokedex. Digging around the Underground will yield Leaf Stones, Thunder Stones, Water Stones, Fire Stones, Moon Stones, and Sun Stones. Our backpack is full of dozens of each and there's no reason yours shouldn't be as well.
Every now and then, you'll come across fractured shards. There's a house on Route 212 that has a lady that will give you a weather effect TM in exchange for ten shards of one color. You'll get Rain Dance (TM18) for blue shards, Hail (TM07) for green shards, Sunny Day (TM11) for red shards, and Sandstorm (TM37) for yellow shards.
Elemental Hold Items
You can also find Damp Rocks, Heat Rocks, Icy Rocks, and Smooth Rocks in the Underground. These items extend the effects of the weather moves listed above. For Pokemon trainers that like building teams around weather effects, these are essential. No Rain Dance team should be without a Damp Rock, no Sunny Day team should be without a Heat Rock, and so forth.