From Bulbapedia, the community-driven Pokémon encyclopedia.
In the core series games, a hole is a special tile found in various multi-floor locations that when stepped on, will cause the player to fall down onto the floor below.
In Pokémon Red, Blue, and Yellow, Strength puzzles involving holes are found in the Seafoam Islands and in Victory Road. In both areas, as part of the puzzle, the player is required to push boulders down the holes, and then follow them by going down the hole themselves. In the case of the Seafoam Islands, the boulders block off currents, eventually leading the player to Articuno. (When falling down holes that lead to water, Surf is used automatically.)
In Pokémon Gold, Silver, and Crystal, there are holes in the Ice Path and in the Blackthorn Gym. Like in Generation I, these are part of Strength puzzles where the player must push boulders down the holes so that they land in specific positions on the floor below, allowing them to properly traverse that floor and eventually complete the area.
A single hole also appears in this generation's version of Victory Road. Unlike the previous generation, there is no Strength puzzle here; the hole simply leads to an otherwise inaccessible area behind a ledge containing TM26 (Earthquake) and a hidden Max Potion.
In Pokémon Ruby, Sapphire, and Emerald, a variation of a hole tile appears: a cracked tile. These cracked tiles appear in the Sky Pillar, Mirage Tower, and Granite Cave. Cracked tiles can be crossed if the player has enough momentum, which is only possible when riding on the Mach Bike at full speed. Once cycled over, they turn into regular holes that will drop the player through as expected if stepped on. If the player moves over a cracked tile without the necessary momentum (such as by walking or running over them, riding the Acro Bike instead of the Mach Bike over them, hitting an obstacle while riding, or otherwise stopping or slowing down), it will turn into a hole before the player can fully cross it, and they will fall through. Any holes created from cracked tiles will reset back into a cracked tile when the player leaves the room they are in.
There are also holes of the regular kind that appear in the interior of Mt. Pyre. If the player walks into them and falls to the floor below, they can reach an otherwise inaccessible area that allows them to obtain the Lax and Sea Incenses.
Additionally, the Sootopolis Gym features cracked ice tiles as part of its puzzle, which turn into holes after they have been stepped on twice (causing the player to fall through the second time).
In Pokémon FireRed and LeafGreen, holes appear in the same location as in the earlier Kanto region games, in the Seafoam Islands and on Victory Road. Cracked ice tiles also appear in Icefall Cave.
In Pokémon HeartGold and SoulSilver, holes appear in the Ice Path and Victory Road the same way as in the Generation II games. However, the holes in Victory Road now must be entered as part of the path to complete the area. (Strength boulders also now appear in Victory Road, but as in Generation II, their puzzle does not involve the holes.)
In Unova, holes appear in Relic Castle, leading down to the next level of the ruins. Unlike previous games, these holes are surrounded by special sand pits. If the player runs across the sides of the pit, it will trap them and the sliding sand will drag them to the hole in the center. Walking across the sides of the pit allows the player to cross them successfully.
Additionally, there is now another type of hole that the player cannot walk into and that does not appear in multi-floor locations. These are exclusively used for Strength boulders, which are pushed into the holes so that they can be walked over, usually allowing access to a new area or a shortcut through the area. Once a boulder has been pushed into such a hole, it will remain there permanently even if the player leaves and re-enters the area.
Strength boulder holes appear in the Generation VI games the same way as in Generation V.
In Pokémon Omega Ruby and Alpha Sapphire, the Sootopolis Gym continues to feature cracked ice tiles as part of its puzzle, although now, once a section of the puzzle has been solved, the cracked tiles on that section become unbreakable. Mach Bike-required cracked tiles no longer appear.
Holes no longer appear in Alola in the same way as they did in the previous games. Along with Strength having been replaced with Machamp Shove, the Strength holes are now gaps in the ground which boulders can be pushed into to get across.