Make a portal to the Nether, which requires you to have 18 obsidian. Build a square structure (for example, five blocks high and four blocks wide) and then use a flint and stone to light the center. Now have a second person join the game and enter the portal. Once the second player is within the Nether, the first player should then destroy one of the obsidian blocks from which it was constructed. Then the person in the Nether should pass back through the portal. A random portal will now appear in your world, containing another 18 obsidian that you can mine. Obsidian is indestructible and thus very useful. You can repeat the process outlined above as many times as you like.