No you do not need wifi to go into the underground tunnles but you can use ds to ds action and play with your friends in the underground and trade and stuff.
Chat with our AI personalities
Yes, yes you do.
You can trade, battle, and have voice chats with your friends over Wifi in Pokemon Diamond/Pearl, but to do so you need to know your friends' Friend Codes, and your friends need to know your Friend Code.
You need a wifi router or a Nintendo wifi usb
If you are referring to secret bases, then a WiFi connection is needed.
yes if u have a Pokemon pearl or diamond trade the Pokemon from the palpark then get another ds put pearl or diamond in then the platinum in the other ds and trade at the Pokemon center at the top