The Buried Relic is, to me, one of both the most interesting, and annoying, dungeons in the Pokemon Mystery Dungeon franchise, specifically in Rescue Team DX
To start, it's a 99 floor postgame dungeon. There are no restrictions on it, unlike most 99 floor dungeons where you're either set down to the starting level or, in the case of Silver Trench, is a water dungeon, which means no natural food spawns increasing the overall difficulty pretty significantly.
Buried Relic, on the other hand, is a dungeon with the quest to find, and recruit, Mew. First you have a fight vs each of the three Regis, which you get fully healed after each, and after that you have the ability to find Mew. The catch is that Mew only spawns on 6 floors of the entire dungeon, one of which is the last legitimate floor (99f is a treasure floor rewarding you for your efforts in beating the dungeon), with no guarantee of it even showing up on the given floor. Along with that it spawns with Transform as a move so you can very realistically run into it without knowing it, and its base recruit rate is only at 8.2%, not insanely low but annoying when you're not 100% able to set it up right and miss out because of it.
In theory, it's an endurance run, with the goal being to find Mew and then get out. In practice, it's a laughably easy endurance run, with food items everywhere and enemies that barely tickle, at the start because the difficulty is meant to scale, and at the end the other gimmick of the dungeon comes to bite it significantly. Said other gimmick is that the dungeon is one of very few with buried items in the walls. Specifically vitamins, and several on each floor. Because of this, even if the enemies are scaled on the last floors to be actually difficult, by the time you reach them your stats are so insanely buffed up that they become jokes.
Having said this, I will admit I do actually like the quest to find Mew in theory, especially in the original version where Mew can actually spawn on any floor and just has higher rates on the floors it was made exclusive to. In addition, the addition of the Strong Foes in Rescue Team DX with shiny chances is neat, as Buried Relic's is Ditto, with the highest chance in the game of encountering a shiny at, I believe, 50%. The reference to a fairly popular fan theory is pretty neat, and having a long dungeon with a realistic chance of encountering a shiny Strong Foe, especially in a dungeon where your goal is recruiting a difficult to recruit pokemon to begin with, is cool. I just wish the rest of it were a bit more challenging. Maybe i just overprepared for it though, idk.
Also apple traps are the bane of my existence, seriously who greenlit these.