Eldiw · shrine
Diving Cave
Diving Cave is a Dielebi Shrine located deep within Gravity Tunnel (reached via a ladder in the top-left of Gravity Tunnel's Underground, then following the path right/up). With Dielebi in the party, it opens onto an alternate dimension containing a retro Lugia.
How to Reach It
Deep within Gravity Tunnel — from a ladder in the top-left of the Underground, head right, stay right, then go up.
Story Relevance
One of the Dielebi Shrine sites; requires Dielebi in the party to reach the Lugia encounter.
Source: pokemon-xenoverse.fandom.com (Diving Cave and Dielebi Shrines articles, raw wikitext); page marked "Temp page," high confidence.