I've heard there are concerns about disturbing the soil because it can trigger responsibility for dealing with the pollutants left by early valley hardware companies such as https://yosemite.epa.gov/r9/sfund/r9sfdocw.nsf/ViewbyEPAID/C...

That is an excellent point and a great way to find yourself the proud owner of a Superfund site.

Mountain view borders the coastline of the bay. I just used a tool[1] to find the level above sea they are, and it shows ~ 8 ft. I imagine there are problems building below sea level when you are only a few hundred feet from the coast.

1: https://www.daftlogic.com/sandbox-google-maps-find-altitude....

Normally next to the ocean is not a good place to dig deep. But I guess with enough money you can solve most problems.

Yeah Manhattan is a good example of going deep but Mountain View sits on soft soil rather than bedrock.

Earthquakes. You'd be hard pressed to find a building in California with a basement. Say nothing of even a few stories down.

Sure, it can be done, but it's not that safe and requires a lot of extra expense.