Hello, everyone. I'm here to explain this feature:
Alright, I will tell you what this feature is about, and give you a little spoiler.
First, in specific places, you will be able to find small holes in the ground, that are not hidden, so I made this sort of easy for you. Every single hole is called the Underground... But, is there a reason for you to go underground?
When you go to the Underground, there will be a little path, so follow it. A little further down this path are some trainers. Battle them to continue. Although they are quite easy, they eventually become bothersome.
Continue down the path once you finish these trainers off, and there will be a little entrance. Enter it. This is where Flash comes in handy. I would recommend beating the first Gym Leader, obtaining Flash, and then coming back to this place.
Anyway, when you're inside, there will be a small maze. Finish it up, and then at the end, there will be a key. These keys are important to collect if you want to enter a certain area of the game, so if you're bored you can go to.
But the problem is, you have to find every key, and there are quite a few of these Underground holes scattered across Malto.
Once you've found every key, there will be a larger Underground hole. Enter this. The setting will some sort of more older, an ancient feel. Walk down the path, and there are no trainers. When you reach the area with the door, the door is much larger, and has interesting designs on it.
There is a Keystone on a Tablet in front of this door. With all of your keys, you insert them into the Keystone. A small tremor starts, the Keystone falls to the floor, the Tablet becomes visible, and the large door opens. Ancient writing is on this Tablet that is unreadable. So, you pick up the Keystone and Tablet just in case it becomes useful.
You enter the door, and a large city is in view. It looks rather abandoned, though. What could this city be?
Here's a little hint: it's a place in Pokémon Colosseum. And no, it's not a place that sunk into the ground, it's always been there. (A few hundred years ago, though.)
Oh, lastly, a certain spirit inside a tomb is catchable in this place.
Phew.
