No way to tell????
Gee they ARE digging around existing tunnels- hint hint, nudge nudge
And they have been digging tunnels under NYC and the rivers for over a century , including aqueducts, railway, subways. SURPRISE !!! We got this thing called boring and soil sampling.
Any size hole you want 3 inches to 24 inches, and you can look at the layers, actually test the dirt, take undisturbed core samples, see where the water is - and it's been around for decades.
And it's done from the surface, right thru the pavement if you like.
Gimme a break.
In the Tehachapis there are a lot fewer, but enough to know about deadly gas, fracture zones etc, and also tons of seismic soundings - the ones the oil explorers use to identify underground strata before they drill holes. Not to mention the gross geologic studies- another century of experience from USGS and CA geology folks. Enough info to identify the scary zones.
AND, oddly enough, legit estimators are supposed to factor in costs for unknowns, and add contingencies for the really really unknowns.
Estimators for the public trough are canny enough to ask " What do you want it to be?"
No excuse for 100% error [ well so far] except for criminally /stupid
But if you want stuff like this for your grandkids, just write a blank, signed check out of your saving/retirement account and send it to CaCaHSR- makes as much sense as 'oh it's okay to be _____ cuz I like trains', but that might make the nonsense more personal.
Hey it's gonna be so wonnerful that lying cheating and stealing justify it.

Too bad the shysters haven't tripped onto this- what a field day and big $
It's called informed consent.
Take any prescription meds?
See the ads for same on TeeVee, and the shyster ads to get money from the drug cos?
Notice that the fine print includes words like " studies show that less than .5 % folks who took this DIED."

How about the promoters of these scams, after they say we really NEEEEED this for our grandkids, add a statement such as
"We haven't a clue about how much it will cost, but it probably will be 50-200% more than we say, and we have the track record to prove it"

It would be really nice to hear ' We were careful with our estimates and as it turns out the honest contractors we somehow managed to hire used their knowledge and skills to bring the project in under time alloted and we have money left to give back to the state treasury"
Almost as good was the fix of the Santa Monica Freeway after the quake.
But if we'll be dead anyway, shoot let grandkids pay for it, instead of something that would have been more cost effective, like a few improvements for the Coast line.
I understand one can still buy a Brooklyn Bridge, or swampland near Benicia- I know a guy