Where is the key to save silva in the under on Pokemon Colosseum?

In Pokémon Colosseum, the key to save Silva is located in the under area called the Under Coliseum. After defeating the Cipher Peon, you can find the key in a small room accessed through the back entrance of the Coliseum. Look for a crate in that room to discover the key. Once you have it, you can return to Silva's location to free him.