An oubliette (from the French oubliettes (noun plural)) was a form of dungeon which was accessible only from a hatch in a high ceiling.
Another form of oubliette was a shelf in a long steeply sloping tunnel leading down to the moat or to the sea. Once cast down the tunnel, a victim would either slowly starve or cast themselves further down to drown.
The earliest use of the word in French dates back to 1374, but its earliest adoption in English is Walter Scott's Ivanhoe in 1819: 'The place was utterly dark—the oubliette, as I suppose, of their accursed convent.' (OED) There is no reason to suspect that this particular place of incarceration was more than a flight of romantic elaboration on existing unpleasant places of confinement during the Gothic Revival period.
At Leap Castle in Ireland workers discovered an oubliette. There are spikes at the bottom of this shaft, and when workers were cleaning it out, it took them three cartloads to carry out all the human bones at the bottom. A report indicates that these workmen also found a pocketwatch dated to the 1840s amongst the bones. There are no indications of whether or not the oubliette was still in use in that period.
There is an excellent example of an oubliette at the chateau in Meung-sur-Loire near Orleans in France. This consists of a submerged structure close to the castle. There is an opening at the top which reveals a large circular stone-clad pit, approximately twenty metres in depth, approximately five metres across, with sheer walls. It has a central hole in the floor, a pit within the pit, the lower pit being used for excrement and dead prisoners.
One example of what might be popularly termed an "oubliette" is the particularly claustrophobic cell in the prison of Warwick Castle, in central England. The access hatch consists of an iron grille secured by a hasp and (now) padlock.
An oubliette of sorts is featured in Cat's Cradle by Kurt Vonnegut, Jr. Though described as a dungeon and associated with other forms of torture, it is outfitted with some life-saving devices and serves as a severe-weather shelter following the introduction of ice-nine into the ocean surrounding San Lorenzo. The novel's narrator laments that "the creature comforts of the dungeon did nothing to mitigate the basic fact of oubliation