Dungeon crawling
Not to be confused with Dungeon Crawl the Roguelike
The primordial swamp of roleplaying: "There be treasure and monsters in that dangerous place. Have fun!"
Players have been pretending to raid mythical dungeons since the 1970s, and will probably be doing so for as long as fantasy roleplaying is a thing. Why? Well, the obvious reason is that this is a fairly logical thing for any kind of Adventurer to do: go clear out the monsters, and find treasure, thus unifying the Paladin (protect the nearby town!) and Thief (steal all the treasure!) archetypes in one easy handwave.
Tabletop Roleplaying in general owes its origins to Dungeon Crawling: The first effective game of Dungeons and Dragons is generally held to have been a Chainmail scenario about infiltrating a hidden tunnel beneath a castle.
A dungeon, for the purposes of this discussion, does not have to be below ground (although it helps): Plenty of Dungeon Crawls involve jungle ruins and aboveground fortresses. What matters is the "room by room" or "floor by floor" nature of the game; each room is its own encounter, with what happens in one room or floor not much effecting what the next room/floor looks like. Some authors try to have themes and logic and shit, but that is usually a waste of effort: The true dungeon crawl can involve goblins next to undead next to a dragon's nest next to a temple to an evil god next to a wizard's hideout, with no explanation of how it came to be nor why they don't kill each other, and be better for it.
Hell, you don't even need to be a roleplaying game to do a dungeon crawl; board games, such as Dungeon, Decent, and Munchkin have done the concept with varying degrees of quality.
Naturally, as this is a fairly simple concept to implement mechanically, many early video games flocked to the concept of a dungeon crawl, or the closely related "hack and slash" roleplaying . Many "fantasy" video games still love an optional dungeon crawl, as it provides some fairly easy to implement exploratory gameplay that requires little to no explanation as to why you'd go there nor what impact it has on the surrounding world; for the former, "because there's treasure there" is sufficient, and for the latter, "it's out in the middle of nowhere" or "they keep to themselves" will suffice. (One of the most notable branches of the /v/ dungeon crawl is the Roguelike. See more there.)
"Hack and slash" is a closely related concept, but differs in that it tends to focus almost exclusively on combat. A dungeon crawl is set in one location, and has, among other things, more exploration, traps that don't involve combat, and usually more puzzles, although the two can frequently overlap. A hack-and-slash campaign can focus more on going from place to place, killing monsters related to the BBEG, and looting their corpses. In other words: no diplomacy, no exploration, combat only, final destination.