The city of Buenos Aires is considering a new public zoo for the Puerto Madero Nature reserve, but with a catch: it is to be a vertical zoo.The Puerto Madero Nature Reserve itself is a piece of man-made nature: a natural preserve from a converted landfill. Similarly, our proposal for the vertical zoo is a fusion of artificial and natural, a habitat for nature grafted onto a man-made landscape.Perhaps the most important aspect of the project is its transformability, allowing the zoo to act both as a walkable, artificial mountain serving as public zoo, and luminous display case, an urban icon for the city of Buenos Aires skyline. Its mixed-mode transformable capacity allow the structure to harmonize with the seasons, becoming an outdoor urban attraction in the summer, and an animal display case in the winter. Situated just off-shore, the structure?s effective footprint in the reserve is zero, preserving the maximal amount of the environmentally sensitive area. Furthermore, the thin-slab allows daylight and ventilation to reach every floor, as opposed to a more traditional tower typology.The experience of any zoo relies on a meandering experience in which visitors are free to roam about and make their own adventure. But in a tower, one is restricted by limits imposed by vertical circulation. The artificial mountain allows for a far greater degree of horizontal movement and inter-connectedness between levels while still providing the ample vertical space as would a tower.The zoo is composed of a permanent support structure, supporting fixed programmatic elements such as administration, offices, auditorium, observation decks, and cafeteria, while the lower, movable levels are composed of individual, modular units comprising zoo and garden spaces. Each of the modular units is individually removable via the attached gantry crane, allowing the entire makeup of zoo animals and garden plant-life to be re-configured as desired. With this alternating garden / zoo pattern, all the food necessary for its own animals as well as indigenous endangered plant species can be grown by the zoo itself.