At the bottom of the surface layer in Area Zero exists a mysterious cave. This is often unofficially referred to as the Secret Cave and it can be a bit tricky to find, as it is quite hidden! However, despite being tricky to find, it is home to a very strong species of Paradox Pokémon: Roaring Moon in Scarlet and Iron Valiant in Violet.
Where to Find the Secret Cave
From Research Station #3, take a left and continue walking forward, heading towards a large tree in the distance. Shortly before reaching the tree, pay attention to your left, as there will be some boulders to the left.
You can actually walk between the boulders to find the entrance to the Secret Cave! Nice!
What’s Inside the Secret Cave
If you’re visiting the Secret Cave while joined by Arven, Nemona, and Penny, you won’t be able to explore the entire cave, and you’ll also presumably be less likely to encounter the rare Pokémon inside, as that seems to be tied to the Pokémon breaking out from the Zero Lab (which you can do soon enough, don’t worry). Because of that, it’s best to return here after beating the Final Battle and seeing the credits, as you’ll be able to ride on Koraidon/Miraidon and climb up to areas you couldn’t explore earlier.
That being said, this is still a very good location to find wild Chansey at level 55, making them a very good way to earn Exp. Points, especially if you boost their encounter rate with an Encounter Power Normal Lv. 1–2 sandwich. You can do this even before beating the game.
In the center of the large room, you can find a Timid Mint and TM138 Psychic Terrain. There are also quite a few useful berries as sparkling items on the ground. Once you can ride on Koraidon/Miraidon again, you can climb up the rocky ledges in the back to find TM170 Steel Beam.
How to Find Roaring Moon (Scarlet)
If you’re playing Pokémon Scarlet, the Secret Cave is the only place in the game where you can find wild Roaring Moon, so you’re going to need to visit this location in order to complete your Pokémon. However, Roaring Moon is quite uncommon to find on its own throughout the cave, so your best bet to encounter it is to boost the encounter rate of either Dark- or Dragon-type Pokémon by using sandwiches.
You can’t picnic inside of Area Zero, so you’ll need to make a picnic just outside of the Zero Gate (the place where you fly to). Making a Dragon-type sandwich is ideal, as it will attract less other Pokémon, but you’ll still see the likes of Zweilous (Scarlet-exclusive) and both Gible and Gabite being boosted. This can be problematic if you’re Shiny Hunting for a Shiny Roaring Moon, unfortunately, but if you’re just looking for a regular one, you should have no trouble finding one.
How to Find Iron Valiant (Violet)
Just as Roaring Moon is exclusive to Pokémon Scarlet, the majestic Iron Valiant is exclusive to Pokémon Violet, and you can rarely find it wandering around the Secret Cave. To improve your chances of encountering it, you’ll want to eat a sandwich that boosts either Fairy- or Fighting-type encounter rates—since there are no other Fairy-type Pokémon in the cave, eat a sandwich with Encounter Power: Fairy Lv. 1–2 and you will find it right away.
You can’t picnic in Area Zero, so you’ll have to make your sandwich outside of it, such as at the Zero Gate (which is where you fly in at). You can also just buy some food that grants that Encounter Power as well, such as the Grilled Rice Balls from Chic Kebab (south-center area of Mesagoza) or Fruit Punch from the Go-for-Broke Grill. These both give Encounter Power: Fairy Lv. 1, which gets the job done.
If you’re Shiny Hunting for a Shiny Iron Valiant, you’ll want to make a sandwich with both Encounter Power: Fairy Lv. 3 and Sparkling Power: Fairy Lv. 3, and you should have tons of Iron Valiant spawn in, although you’ll need to pay careful attention to find a Shiny one; the good news is you won’t have to worry about other Pokémon being boosted due to the sandwich, which is an advantage Iron Valiant has over Roaring Moon.