The design of the Santiago HPP on the Santiago river in Ecuador, realized by the Comisión Federal de Electricidad (CFE) of Mexico, consists of a 190 m high RCC arch gravity dam connected to an underground powerhouse equipped with 6 units with a total installed capacity of 3’600 MW. During the design phase, Lombardi has been charged with a technical assessment of the feasibility study and the basic design by the final client, Celec EP of Ecuador. The proposed design is very challenging in many aspects, like the installed capacity (the project will be the 19th in the world and the 3rd in South America), the magnitude of the flood (PMF=21'000 m³/s), the seismicity of the area, geologic challenges (such as the large unstable area on the right river bank downstream of the dam) and the accessibility of the area. Lombardi effectively collaborated both with CFE and Celec to optimize the design of the plant, working both on the general layout and on the details of the design of the individual components like the dam and all the underground works. The services provided by Lombardi required the collaboration of many senior experts in different technical fields.