Unlocking the post-game content in old Pokémon games has always been the ultimate reward after playing through the entire story, bringing new mysteries to solve, places to explore, and even some hidden secrets. The Altering Cave is such a weird oddity, though; if you thought there was more lurking beneath the surface about this place, you wouldn't be that far off from the truth. It is home to an event long forgotten by its creators shortly after the games were released, dormant, awaiting the right trigger to activate it, one that will never come.

Over the years, multiple fixes and external tools have been developed to aid players in discovering everything the Altering Cave hides. If you are interested in this event but lack the knowledge to make it work or simply can't find the time, this tool will help you find the wild Pokémon you're after in a few simple steps.

More information.

Unlocking the cave and hunting.

• In FireRed and LeafGreen, the cave is not available until after having access to Six Island, which is unlocked after receiving the Rainbow Pass from Celio. To obtain this item, you will need to enter the Hall of Fame, obtain the National Pokédex, and do Celio's questline about retrieving Mt. Ember's Ruby in order to complete the connection with Hoenn inside the Pokémon Network Center (the orange building on One Island).

  After fulfilling the previous requirements, the cave can be found on the northernmost part of Six Island, on a small piece of land called Outcas Island.

• In Emerald, it will also appear after entering the Hall of Fame, but no further requirements are needed. The entrance of the cave will appear on Route 103, near the shore on the east side of the water.

All of the Pokémon that can be found here (or their evolutions) are also available in Emerald's Safari Zone expansion and as Shadow Pokémon in Colosseum, which work as alternative ways to hunt these targets.

The pros and cons of hunting them in each of these locations are:

• Altering Cave: the Pokémon will have "Altering Cave" as their caught location in their data and may also have the chance to come with a special item. The hunt process is more straightforward, and there's no chance of phasing. The targets are not a common sight in the Kanto games. Unofficially activated, so it depends on your definition of "legit".

• Safari Zone: the Pokémon will be caught in a Safari Ball. The encounters can be faster than regular ones, but there's the added risk of having the Pokémon flee. Phasing is possible unless there's any repel trick or ability like Static being used that makes the target a 100% chance to appear.

• Shadow Pokémon: the hunts are significantly slower than the previous two. Shadow Pokémon get a special ribbon after being purified, and most of them are the only ones that can obtain all available ribbons in gens 3 to 7 (or more for those who also appear in the gen 8+ games). It is a common agreement that these hunts feel much more accomplishing than the rest, aside from the fact that some have different colours and they are all displayed with 3D models instead of 2D sprites.

Special items.

In FireRed and LeafGreen, the Pokémon found inside the cave will have their normal chances to hold an item (if any), but in Emerald they will have a small chance to hold items they would not have under regular conditions.

* Shuckle always holds Berry Juice in FireRed and LeafGreen, but it doesn't appear in the wild anywhere except this cave.

Technical information.

Read before continuing.
The following information is just an educational insight into the inner workings of this tool.

The Altering Cave was meant to require a Wonder Spot to activate a Mystery Gift on your save file that would change the Pokémon found inside it, which was never distributed officially. Otherwise, the Pokémon is always set to Zubat.

This Pokémon is stored inside the save file as a value depending on the species. This value is located in different parts of the save file for Emerald and FRLG. If you don't know about the structure of a Generation 3 save file, they are separated into 2 save blocks (one for the current save and one for the previous one). These blocks are also divided into 14 sections each, going from section 0 to section 13. Specifically, section 2 is the one we are interested in for the Altering Cave, which contains multiple variables related to the game state. Our value is located in different offsets for each of the games:

This is a 2-byte-long constant whose value ranges from 0 to 8. Each of the numbers corresponds to a different Pokémon. Any value above 8 will result in a Zubat.

In order to modify the current Pokémon, this tool first finds which of the two save blocks is the current one (A or B). After that, it needs to figure out the order in which the sections of the save are positioned, as each time that you save, the sections get "rotated". For example, the first time the game gets saved, the sections will follow this sequence:

And, if we save the game again, the sections will rotate by one position:

The sections will always stay in that order and will never be shuffled randomly. After we know in which position section 2 is, we can start modifying the value of the cave, depending on the version of the game. But just editing the value is not enough, as only this change will show a corrupted save file error, prompting the game to load the previous save. That's where the checksum comes in.

The checksum is a 16-bit value that makes sure the integrity of the data is correct. Each of the sections has this value stored in a footer found at the end and is generated with an algorithm that processes the data inside the section. After we calculate the checksum using the same algorithm and write it on its correct offset, the save is good to go!

Disclaimer: This tool does not activate the original Mystery Gift event or any fanmade ones; it simply modifies the value of the Altering Cave target without requiring any knowledge about save editing software or custom distributions. If you want to change the Pokémon with Mystery Gift or other tools, you can find more information here.