The Phantom Tollbooth Wiki

The Doldrums as seen in the 1970 Chuck Jones film.

The Doldrums is a place within the lands beyond the tollbooth. It is where Lethargarians live and do nothing all day. People who don't think and/or don't pay attention often get stuck there. It is a cloudy place in the countryside which has hardly any trees or flowers. The grass is not very brightly coloured. There is no wind and the birds sing only boring songs. Nothing ever changes. To get out of the Doldrums, one must think. To be more specific, we call it daydreaming.

In the film, it is a disgusting-looking place where the ground and several rocky spires are covered in greenish-yellow slime and the Lethargarians are its only inhabitants.

In the film, the slime proves to be a challenge for Milo's Car as the sticky slime prevents it from proceeding any further. The Lethargarians also seem to have control over the slime in the Doldrums as during the song "Don't Say There's Nothing to Do in the Doldrums", as the slime swarms over Milo's Car, one Lethargarian was seen waving and beckoning the slime onwards to the car.