Also known as the San Francisco Bay Plan, it was a 20th century campaign to cut off and reclaim a large portion of the SF Bay, turning it into two freshwater lakes. Modeling efforts determined it would have been an unprecedented environmental catastrophe.
They built a giant scale model to test it in the days before computer simulations. You can still tour it.
https://en.wikipedia.org/wiki/U.S._Army_Corps_of_Engineers_Bay_Model
Tom Scott did a great video about that model and this plan.
https://www.youtube.com/watch?v=i70wkxmumAw