In the first aliyah, G-d gives Noach detailed building instructions for building the tevah (ark) and instructions for loading the tevah with two of every kind of living thing in order to perpetuate each species.

Rashi says that it took Noach 120 years to build the tevah so that people would see Noach building it, and ask why. He would tell them that G-d was going to bring a mabul, and this would give the people the opportunity to repent.