A bottomless pit, as its name implies, is a pit that has no identifiable bottom. They are a common hazard in many video games.
Depending on the game, when a player character falls into a bottomless pit, they may lose either a life or health and/or be forced to restart from the last checkpoint, often regardless of invincibility or any other power-ups the player may have obtained. In some games, this can be avoided through a special item. e.g., in Mega Man 7, the robotic bird, Beat, can prevent Mega Man from losing a life. As such, the bottomless pit generally serves as a navigational hazard which the player must be conscious of at all times. They may also be used as puzzle elements, by forcing the player to cross them, fill them in, or otherwise circumvent them by some pre-determined means, and thus restricting access to what is on the other side of them until that challenge has been completed. In fact, sometimes in 2D side-scrollers, a floor below the on-screen area could be hidden by a seemingly-bottomless pit; such features usually hold secret items. Few bottomless pits have a safety net, though very rare. e.g., in Sonic Adventure 2, the level Pumpkin Hill, played by Knuckles, is very vast and open, and thus it is very easy to lose a life. If you fall into the bottomless pit, it will let you hover for just a while, enough time for you to return to solid land.
In first-person shooters, pits are generally not bottomless and fall damage may instead result in damage or death. Other alternatives to the bottomless pit include water, lava, spikes, quicksand, and crushing, all of which may vary in damage. Players are not usually able to jump out except by using cheat codes.
In the Zork games, the early versions had bottomless pits in rather unlikely areas, such as attics, and the aforementioned pits would only appear in pitch-black areas, vanishing as soon as a torch was lit. These were later replaced with grues.