[21twelve]

http://nwgeology.wordpress.com/the-fieldtrips/glacial-erratic-field-trips/the-lake-stevens-monster-largest-erratic-in-washington-largest-in-the-us/

Here’s an article with photos of a large boulder (34’ x 75’) that is on the ground surface about 50 miles north of the tunnel.

A large cavern is very unlikely in the geology where the boring is. And it would not “obstruct” the boring - but they obviously wouldn’t be able to construct the tunnel.

Although it would be REALLY COOL to have to drive into the tunnel, go a mile, then drive across a bridge across the cavern, and then into the tunnel again on the other side! Have a “view point” on the edge of the cavern where you could get out and stretch your legs, and maybe have walkways to go exploring. Sort of a Jules Vern thing!

[fso301]

Here’s an article with photos of a large boulder (34’ x 75’) that is on the ground surface about 50 miles north of the tunnel.

Occam's Razor says this is the most likely answer. I tend to expect these boring machines are capable of going through solid igneous rock, albeit at a much slower rate.

[BenLurkin]

Interesting

[PIF]

There are numerous erratics - the largest I saw is near Chincum, WA about 40 miles northwest of Seattle. Not as big, I guess, as that at Lake Sevens but very big.