[Nick Danger]

• in the US nuclear submarines miles deep below the ocean surface
• special magnetometer receivers in the US nuclear submarines miles deep below the ocean surface

The maximum depth for an Ohio-class boomer is around 300 meters... about 1/6th of a mile.

[brityank]

Agree most of this seems to be bull, but the ops depth runs over 1200 feet now; I lost a good friend here:

Thresher sank in 8,300 feet of water, taking 128 crew members with her. The boat had an operational depth of 1,300 feet, more than any other U.S. submarine class to that date, but clearly the hull would have passed "crush depth" well before hitting bottom.
NOVA - 400 Years of Subs