The Sinnoh Underground is also vital if you wish to get the Pokémon, Spiritomb

The main thing that you do in the Sinnoh Underground is Excavating in the walls for various items. After locating them, this is controlled purely with the touch screen. It gives you a Pick Axe and a Hammer to use to try and break through the rocks in order to find 2 to 4 items. However there is a limit on how much stress you can induce on the wall through this before the wall collapses itself. To make things worse, there a good number of rocks that are just there to take up space that can cause a cave in if you keep hitting them

There are a number of items that are found here including all the evolution stones, shards that are used to trade for evolution stones, the different plates used for Arceus and various items such as Revives and Hard Stones

You can also find all of the fossils here after you get the National Dex; Dome Fossil, Helix Fossil, Old Amber, Claw Fossil & Root Fossil. Also you can get the Armor Fossil & Skull Fossil depending on your version at any time. However unlike in past games, you can have any number of these fossils on you so you can go revive 20 Omanytes or Anoriths if you do so wish.

The other thing you will find is the currency of the Underground; Spheres. There are 5 different kinds of Spheres found in the underground; Reds, Blues, Greens, Prisms & Pales. However, when you keep getting them, they are not combined and there is only way to combine them and that does cause a loss in the Spheres. What you do is drop the highest amount you have on the ground and keep dropping each subsequent piece onto the ground and then pick it up. You will get the original drop amount plus the extra amounts. Below is a table that states the amounts that are given for the Spheres of particular sizes:

There is another way to get Spheres however, in the far corners of the underground, there are traders that will give you large amounts of gems (in quantities of over 70) in exchange for items you have found. However, be careful as some of the items you find you will only get once such as the Arceus plates.

Secret Bases make a welcome return. Placed within the Underground world in Sinnoh, you can decorate them like in Ruby & Sapphire. To access them, you need to get a Drill from the old man in Eterna City. This Drill is one use so you have to be careful. However you can pick up another one from the hikers for a price

Once you have created your Base, you'll want to kit it out and hidden amongst the underground are a number of Hikers, all of which sell specific items for a certain amount of the Spheres explained above. Each of the hikers sell different things for your Secret Base for a various amount of Spheres, some of which will definitely require you compile them with the above method. Then you simply kit it out when you get inside the Secret Base.

You can also obtain more items with Mr. E Good's Tasks

Secret Bases do have another use however...

Flag Retrieval is a small mini-game that you and a friend can play. Simply put you need to go to your friend's secret base to get a flag. However your friends are capable of placing traps such as strong wind, boulders etc. to stop you from progressing. The first back wins the game.
The Winner manages to get upgrades to their own Secret Base so they can place more items in it.
There are multiple different levels in the Flag Retrieval, the highest being the Platinum level, obtained by capturing 50 flags from your friend's bases. If you have this ranking, you can remove all of the annoying rocks in your Secret Base

Next to the Pokémon Center in Eterna City is the man that gives you the Explore Kit. Once you have the Explore Kit however, he gives you many tasks that you are to do in the Underground. Do the missions of his, and he'll give you many things. Below is a list of his missions;