These are traps placed in the dungeon. Pits can be concealed, visible, fake, and of course open or closed. Some tend to hide useful items and a teleporter to return you to a different location, where as others are places of no return, and reloading may be the only way out.
Falling/jumping down pits does cause damage.
A rope or grapple can be used to descend safely down a pit, if of course the pit is only one level deep.
The grapple may be used to ascend pits. Remembering not all dungeons have these items.
Placing a Pit
Pits are a floor item, this means they can be found in the add floor objects heading of the RTC editor. A floor tile must be placed on the level below the pit in the matching location. For aesthetics a pit hole in the ceiling may be used. Currently these are not essential when creating a dungeon and will not result in any warnings, this is also the case if a pit hole ceiling does not lead to a pit in the level above.