Forwarded to me by Stewart Brand is an amazing article from Archaology Magazine on the construction of the Great Pyramids. After analyzing a 25 year old micro-gravimetric study that showed a spiraling sub structure, it was determined that the pyramids were built with a spiraling ramp as part of the internal structure.
A microgravimetry survey of the Great Pyramid in the 1980s yielded the enigmatic image at right. Less dense areas (indicated in green) seem to correspond to an internal ramp proposed by Jean-Pierre Houdin (diagram). (Dassault Systemes; Courtesy EDF)
While many have asserted that a ramp, either spiraling or straight, may have been used, this study gives evidence of this ramp as an internal structure that was then covered up as the pyramid was completed.