“Oubliette” is a French term derived from the verb “Oublier” which literally translates to “to forget”. The term describes a dungeon with an opening only at the top. The victim, once thrown into the oubliette, was considered forgotten by the outside world and remained so presumably forever.

That seems like such a direct method of handling ones enemies: toss ’em in a hole and forget ’em. As a child I had my own version of this although mine was more horizontal in nature. My brothers and I practiced the extraordinarily stupid ideas of “dare” and “double-dare”, ostensibly as a way of establishing a masculine pecking order or some weird bravery index.
Once, after a heavy rain, we were out wandering the wet and soggy Maryland countryside looking for mischief or other marginal adventures. We noticed a culvert under the road which was partially filled with rushing water and we thought it would be fun to crawl through this small tunnel and emerge wet on the other side of the road. Ah, the simple joys of youth.

it was a short journey of around 20 meters and of course I, the youngest, was the last go. At about the halfway point I noticed the light at the end getting smaller and smaller rather than larger and larger as I crawled toward it. My brothers thought it would be the coolest thing ever to block the far side and have the rushing water back up in the tunnel with me inside! As the water rose and the darkness closed in menacingly I assumed that this was basically it, game over — a forgotten boy under a road. They eventually pulled the blockage aside and the water gushed out carrying me along. I told them I “enjoyed the ride” so they would feel bad at having missed out.
This experience either cured me of claustrophobia or gave it to me, not sure.