up of walls of different thickness depending on their function. The walls that are thicker, have a load-bearing structure of reinforced concrete inside. The four vertices were preserved as "chambers", where the contemporary and museographic altars are beautifully placed, alluding to those that served to worship deities in pre-Columbian Mesoamerican contexts. Since this section represents the Underworld, it doesn't have much lighting. On the first floor that is also the middle floor, the four vertices were kept as "crypts" and there are four exhibition halls. Among these rooms, the two smaller ones face south, and the larger ones are longitudinal, one being clearly more integrated into the central space. On that same level is the space known as Diego Rivera's Studio. This section represents the earthly world, so its rooms are endowed with natural light, unlike the lower floor. After finishing the spaces mentioned above, the roof was built. The design of the second level forms a "U". In this area, the "crypts" were integrated into the longitudinal rooms, while the small rooms remained, repeating the layout of the lower ones. Although O'Gorman was determined to respect the scheme proposed by Rivera, this was not entirely possible in the case of the roof, since Diego had planned it with a singular lightness. This is demonstrated by two letters mentioned in this regard by the architect Ruth Rivera, Diego's daughter, and by Pedro Alvarado, one of the muralist's grandchildren. In these letters, Rivera highlighted: "the importance of Ajusco for the solution of his project" and how the roof should "rhyme with the nose of Pico del Águila", due to its slender design. Rivera thought of a "light crowning", since "the truncated pyramid" must have "a new body that gives the building a vertical character", which contrasts "with the horizontal character achieved by the building as well as with its surrounding landscape". For the actual technical solution, he thought of a metallic profile drowned in concrete and "the inclined-vertical plane of light sheets of thin marble or tecali, the ceiling itself of pre-cast concrete sheets…or else cast on site with the same procedure." Finally, Diego wanted the roof to end in a lightning rod. The painter also suggested placing a palapa to cover the structure of the rubble. Due to important decisions regarding costs, as well as for assuring the durability and the stability of the construction, Juan O'Gorman's final solution could