It is both wet and generally fine for tunneling. The D.C. metro (subway) is almost entirely underground within D.C. city limits (though it runs mostly above gro

It is both wet and generally fine for tunneling. The D.C. metro (subway) is almost entirely underground within D.C. city limits (though it runs mostly above ground outside of the city) and basements and extensive underground parking garages are the norm (some going very deep). The region being looked at is the Atlantic Coastal Plain and a little of the Atlantic Piedmont Plateau. Below the wet marshy bits it's mostly sedimentary prelithified and lithified rock in the coastal plain area and a complex geology in the Piedmont area. The Piedmont area used to be the base of an ancient mountain range that's since eroded away while the Coastal Plain used to be entirely underwater at some period. https://en.wikipedia.org/wiki/Atlantic_coastal_plain https://en.wikipedia.org/wiki/Atlantic_coastal_plain https://en.wikipedia.org/wiki/Piedmont_(United_States) https://en.wikipedia.org/wiki/Piedmont_(United_States)