The original 1956 plan for the Bay Area Rapid Transit system was way more complex on paper than it is in reality. Designer Jake Coolidge has imagined a universe in which this full plan was implemented. And even though it doesn't involve filling in the Bay or underground rocket trains or pyramids, it does complement world-building for any tales about an alternate reality San Francisco rather nicely.

See Also: San Francisco's crime rate, topographically.

[Jake Coolidge via Laughing Squid]