The Area Zero Underdepths is a large underground expanse filled with large and beautiful crystals and a deep pit of darkness down below.
There are also tunnels connecting different parts of the cavern with Terastal crystals growing out of the rock. As it gets further inside, more and more crystals start to appear until the innermost tunnels are nothing but crystals as an entire large tree is covered in crystal.
Unsurprisingly, a few Pokémon also inhabit the Underdepths along with Terapagos, mostly those related to crystals.
According to the Professor, the Area Zero Underdepths may have been formed roughly two million years ago possibly by seismic activity.
This section is incomplete. Please feel free to edit this section to add missing information and complete it. Reason: Complete list of Pokémon encountered and table
A variety of Stellar Tera Pokémon must be defeated in order to shrink the crystals blocking the way during the events of The Indigo Disk, allowing the player, Carmine, Kieran, and Briar to progress further into the Underdepths. None of them can be caught, and are coded to never be Shiny.
A Garchomp can be encountered and subsequently defeated in order to shrink a crystal in the room prior to one that wraps downwards in a staircase. It cannot be caught, and is coded to never be Shiny.
After Kieran has been defeated, he will attempt to Terastalize Terapagos, causing it to become uncontrollable. The player must battle it with no opportunity to change teams or heal from the end of the previous battle; losing this battle will revert the game back to before the player's battle against Kieran. It must be caught by the player upon its defeat, and is coded to never be Shiny. Additionally, this Terapagos is always coded to be male, despite it having a gender ratio of one male to one female.
In the battle against Stellar Form Terapagos, it has four times the usual amount of health (with a large health bar across the top of the screen in a similar fashion to Titan Pokémon and Starmobiles, which identifies it as Terapagos, the Hidden Treasure of Area Zero), and unlimited PP for all of its moves, but its Teraform Zero ability does not function. Similar to a Tera Raid Battle, it can put up a shield to become immune to many status moves and reduce the damage it takes from attacking moves, and the player will be unable to Terastallize their Pokémon until they charge the Tera Orb by hitting with a certain number of damaging moves, but only two attacks are required to charge it, instead of the three from standard Tera Raid Battles. Unlike Tera Raid Battles, there is no time limit and no cheering, and the player can switch Pokémon and use items from the Bag.
Terapagos will put up three shields over the course of the battle: one at the beginning, and the others immediately after the previous shield is broken. Each shield covers 27% of Terapagos's health bar, and if any attack deals enough damage to break the shield, all further damage from that attack (including from multi-strike moves) is prevented, similar to Pokémon Sword and Shield's Max Raid Battles. When a new shield is put up, Terapagos is cured of any status condition, all stat modifiers to any Pokémon are erased (but other field effects such as weather and Light Screen remain in place), and if any of the player's Pokémon have been Terastallized, they lose that status. The Tera Orb must be recharged before the player can Terastallize a Pokémon again.
In the first phase of the battle, Terapagos is Normal type, and Carmine participates as a partner using her Sinistcha. If Sinistcha faints, the player must continue the remainder of the phase alone. If Sinistcha has not yet fainted when the shield breaks, Terapagos will use Zen Headbutt at that time, and Sinistcha will automatically faint as a result, regardless of any effects that would prevent the move from being used, or would reduce the amount of damage dealt.
In the second phase, Terapagos changes to a Psychic type, and the player must face it alone.
In the third phase, Terapagos changes to a Water type, and Kieran joins as a partner using his remaining five Pokémon. Terapagos will remain Water type after the final shield is broken.
When Kieran is controlling Terapagos, it possesses twice the usual amount of HP and has a large HP bar at the top of the screen that identifies it as Terapagos, the Indigo Disk.
