It has been called 'The chimney' but the purpose of it was unknown.
In the diagram below it is labelled 'shaft'.
The descending passage of the Bent Pyramid entered the first corbelled chamber that was built well below the second chamber. It was narrow and would have housed a ladder to get up into the second chamber.
The above drawing is from Lehner's The Complete Pyramids.
The ladder in the chamber seems to have been about 20 cubits long, as measured from a scale drawing. It passed through the niche door at the top which was 2 cubits wide.
Directly opposite the niche for the ladder was the 'Chimney' feature.
The shaft was vertically below the apex of the pyramid. It was underground and reached up to ground level.
What is not shown in Lehner's diagram are two other key features.
One is the little corbelled window from the chamber above the entrance into the chimney.
In the diagrams and picture below you can see the corbelling of this little window into the 'chimney'.
Secondly not shown is the rectangular pit in the little passage leading into the base of the chimney.
This pit is not under directly under the chimney but well in front of it. These diagrams above show it clearly.
The depth of the pit was 14c and the height of the passage from the chamber floor was 6 cubits. Thus the total height from the bottom of the pit to the top of the passage was 20c. The width of the chimney was 2c.
Now looking at the above picture it would be possible to insert a 20 cubit long 2c wide ladder into the chimney.
If the feet of it were put down to the bottom of the pit (14c) and it could then be tipped forward into the chimney through the slot (6c). Then it could be lifted out of the pit, up the chimney, and stood on the ledge at the base of the chimney.
It is clear that the design of the chimney area totally complies with such an idea.
Assuming the chimney was for ladder, it could logically have been placed in the chimney when it was first built, so why cut a pit ? The existence of the pit means that it was required to be inserted AFTER the pyramid was finished. It may have been symbolic and part of a ritual.
There are references to this in the Pyramid Texts.
The Ladder (or Stairway) we are told led to the destination of the king's ascension. The gods of the Two Enneads (9 + 11 gods) made the ladder and dwelt on it in the northern circumpolar star area.
From the PT :
Utt 480 An ascension text
"They lift up the ladder for the King, they erect the Ladder for the King, they raise the Ladder for the King. Come, O Ladder, come O Ladder, may there come your name which the gods have spoken. He who ascends comes, he who ascends comes. He who climbs comes, he who climbs comes ! He who is lifted up comes, he who is lifted up comes. The King ascends upon the thighs of Isis, the king climbs upon the thighs of Nepthys, the King's father Atum grasps the King's hand and sets the King at the head of yonder gods who are excellent, wise and imperishable. Behold this which has been said to you, you gods, lest the King be not at your head; behold, the King is established at your head as the enduring Bull of the Wild Bulls.!"
Utt 478 Invocation of the Ladder to the sky
Hail to you, ladder of the God . . . Stand up, Ladder of the God . . . Now let the ladder of the God be given to me that I may ascend on it to the sky and escort Re.
As for any spirit or any god who will help me when I ascend to the sky on the ladder of the god : my bones are assembled for me, my limbs are gathered together for me, and I leap up to the sky into the presence of the god the Lord of the Ladder.
Utt 306 An ascension text
There comes to you the gods the Souls of Pe, the gods the souls of Nekhen, the gods who are in the sky, and the gods who are on earth. They make supports for you upon their arms, that you may ascend to the sky and mount up on it in this its name of 'Ladder'.
Utt 530 An ascension text
Hail to the Ladder . . . set your hand on me that I may sit between the two great gods, that my seats may be pre-eminent , that my hand may be taken at the Field of Offerings, and that I may sit among the stars which are in the sky.
A ladder in the 'chimney' could thus symbolically and magically have represented the ladder of the gods in the sky to which the king had ascended and took charge.
It seems probable that it would have been the same ladder used by the King to ascend into the chamber from the lower room. The climbing of the ladder from the lower room could have been a symbolic ascension into the sky.
The lower room was thus 'on earth' and the upper chamber was 'in the sky'.
The 20 cubit ladder was likely to have been twenty steps of one cubit each. One for each of the 20 gods of the Two Enneads. (The Great Ennead was certainly nine gods. The Lesser Ennead were the spirits of Osiris around the country and 11 are listed in Utt 219.) How this ladder looked cannot be known.
From the PTs it is clear that the Ladder and the Two Enneads were part and parcel of each other.
The corbelled window from the chamber into the chimney viewed the Ladder/Two Enneads.
The lower main part of the window into the 'chimney' was at 11 cubits above the floor of the chamber, thus the window viewed the split of the 20 cubit ladder into 9 and 11 cubits.
This mirrors the same split of 90 and 110 cubits in the height of the pyramid directly above.
It is possible that the Bent Pyramid as a whole may have been dedicated to the Ladder or 'Stairway to Heaven' as it is sometimes popularly called.