It is located east of Sophian Island and can be accessed by riding the Union ship from Tilt Village on Mitonga Island. The Undersea Cavern (Japanese: かいていどうくつSeafloor Cave) is located here. It is often a victim of pollution because of its unpredictable currents, large amounts of trash get carried here.
The player first arrives at the Undersea Cavern after being assigned a quest in the Aqua Resort to search for a parcel a Delibird dropped while performing a delivery.
After capturing all 300 Pokémon in the Browser (excluding Pokémon from Extra Missions), a new quest is unlocked where a pair of Pokémon Pinchers at Rand's House ask the player to rescue their leader who has gotten stuck under a boulder at the bottom of the Undersea Cavern. After rescuing Blue Eyes, the player is challenged by a very agitated Lugia while aboard the Union. After capture, Professor Hastings suggests that Lugia was more agitated by the pollution rather than the Pinchers' presence and was only protecting the ocean.