I have vague memories of this story - probably a short or novelette length published in a magazine like Analog in the 50s or 60s. A human space traveler lands on an artificial planet-sized sphere, finds his way inside, finds civilized beings, takes an elevator down a number of levels, gets out and takes a larger elevator further down, where he again finds an even larger elevator and heads down again.
The story, IIRC, had an odd single-word name like Kerplunk or Skiddoo.