What is a "base tunnel"? Definition here.
I keep seeing reference to the Palmdale "base tunnel" here on AP. I thought I'd Google that term, "base tunnel."
Here is what Wikipedia has:
"A base tunnel
is a type of tunnel, mainly a railway tunnel, that is built through the base of a mountain pass. This type of tunnel typically connects two valleys at about the same altitudes.
Some of the best-known base tunnels are (with length, opening and location):
Gotthard Base Tunnel (57.1 km, 35.5 mi, 2016), Switzerland
Mont d'Ambin Base Tunnel (57 km, 35 mi, planned), France and Italy
Brenner Base Tunnel (55 km, 34 mi, proj. 2025), Austria and Italy
Lötschberg Base Tunnel (34.6 km, 21.5 mi, 2007), Switzerland
Koralm Tunnel (32.9 km, 20.4 mi, proj. 2022), Austria
Semmering Base Tunnel (27.3 km, 17.0 mi, proj. 2025), Austria
Pajares Base Tunnel (24.7 km, 15.3 mi, proj. 2012, but currently not usable), Spain
Zimmerberg Base Tunnel (20 km, 12 mi, planned), Switzerland
Apennine Base Tunnel (18.5 km, 11.5 mi, 1934), Italy
Ceneri Base Tunnel (15.4 km, 9.6 mi, 2020), Switzerland
Furka Base Tunnel (15.4 km, 9.6 mi, 1982), Switzerland
Ore Mountain Base Tunnel as part of the Dresden Prague High Speed Railway Line
Base tunnels through the San Gabriel Mountains have been proposed as part of the California High-Speed Rail system."
Now you know.