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.
2.5 billion doesn’t actually seem that bad for a project of this scale. But according to a quick calculation on an internet site it’d be about 26 billion today.
You’d spend at least twice that today just acquiring the land rights via eminent domain.