underground?

I take it that you are not that far into the game? Because if you were you would know what the Sinnoh Underground was. But do not worry, it is not a big deal at all, I will help you.

The Sinnoh Underground is an area where you can link up with your friend and just hang out, or dig up fossils. You can also play tag with your friends, and play steal the flag. You can also do things such as make a secret base in the wall, and find fossils. It is really fun, in order to access the Sinnoh Underground, you need to talk to the Underground man in his house in Eterna City.

Hope this helps.

You have to get some supplies. These supplies can be obtained from a man in Eterna. He will give you the explorer kit. To access the underground use the explaorer kit outside and you will be transported there. You do not need a friend to go to the underground but there are some things that having a friend would help do. In the underground you can find many items. These include stones, shards, fosils, revives, max revives, plates, spheres, and star pieces.
Spheres in the underground are like it's currency. If you find a hiker you can talk to him and exchange spheres for goods. You could also get a drill. The drill helps you create a secret base in the underground. You can adorn this base with furniture, dolls, and tons of other things. All this and so much more is in the undergroound so why don't you check it out.
-Hope this answers your question.

Underground is like another section in the game. U have to get the Underground Kit from the man in Eterna city. His house should be beside the Pokemon Center with a sign in front of the house. It might be beside the bike shop. It's one of those two.

You use the explorer kit that you receive from a man in Eterna city anywhere you want to use it aside from on water or grass. And no you do not have to have a friend with you to go underground.