My Not-So-Hyper Loop
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There'd still be some major engineering involved; ATW the deepest transit tunnel at present, the Seikan Tunnel in Japan, is only about 140 meters below the seabed and 240 meters below sea level.
On the other hand, there's a mine in South Africa that's nearly 4km deep, and people work down there for extended periods of time. The mine has several hundred km of tunnels, too.
You could of course hook the system up to a power grid that could be getting power from solar panels, but Elon's description sounded like he intended for the panels to be part of the system, which would require it to be above ground