I'm just imaging a 150 way 3-d lane merge at the other end :O

When imagining a 150 way merge I can't help but feel like it'll pollute enough to make nearby trees crumble in a matter of days. 150 ways at approximately 1 car per second (a very full highway) and 1.1 people per car (I hardly know anyone who carpools), there's 1501.1120 = ~20 000 people every two minutes. Trains can operate at the same frequency, carry a few thousand each and pollute a lot less -- heck, electric ones can draw from solar panels, reducing pollution to near zero. Give it ten tracks and you have equivalent capacity to 150 stinky lanes.

Musk's idea was that gas-powered cars would not be driving through tunnels themselves, but on electric sleds, which makes the tunnels as non-stinky as with electric trains.

I think it's rather scary how thin the mantle is in many places. The oceanic crust is just above 10km deep, I'm sitting on something like 15km. That's.. not a lot. I can cycle that in less than an hour.

I'm digging the tunnel love shown here.

What's an earthquake like when you are on the 12th (or is it -12?) floor of an underground building?

That's among the safest places to be. There's a whole section about it if you read the article linked at the top of the page.

The problem is that there are only positive examples in the article. Of course that tunnels may collapse or be damaged during an earthquake.