[jqm]

I was hoping for an explanation of how they are able to pinpoint exactly which direction to dig and come up. I mean.. coming up into a 2 foot square from 35 feet underground from a mile away isn't something you guess at. GPS signals don't work underground do they?

[willyt]

Laser measurement systems. e.g. http://ptd.leica-geosystems.com/en/Total-Stations-TPS_4207.h...

[hamiltonkibbe]

Survey equipment. RTK GPS for position reference and heading on the surface, and lasers. Lots of lasers.

see e.g. http://www.leica-geosystems.us/en/Tunnel-surveys_1693.htm

[joshuapants]

> GPS signals don't work underground do they?

They probably wouldn't. But you also don't necessarily need GPS to do this. Have someone survey it topside, use that data and a compass underground.

[dsmithatx]

If you know the exact direction at the opening and, the tunnel is in a straight line a laser pointer should work fine. Just keep making sure it hits where you are digging. Reading the entire story it sounds like that is all they were doing.

[mzs]

GPS does not work underground, you can for some impressive distance get away with a long cable and an external antenna, but there are also GPS repeaters available.