How did the Vietnamease build such dynamic tunnel systems?

Question: How did the Vietnamease build such dynamic tunnel systems!?

like this pic
http://news!.bbc!.co!.uk/olmedia/720000/ima!.!.!.Www@QuestionHome@Com

Best Answer - Chosen by Asker:

necessity is the mother of all invention!. To be honest, Vietnamese had learned to dig excellent and extensive tunnels as far back as the Chinese domination, prior to the French coming in the 1780's!. Throughout the Vietnamese war with the French, they dug tunnels, as well!. However, the US war in Vietnam forced the tunnel digging efforts into a new level of sophitication!. Us air power and fire power made the necessity of tunnels (for cover from fires and from observation) grow many times over!. While the US built extensive, sophisticated base camps above ground, the Vietcong (and NVA) did the same below ground!.Www@QuestionHome@Com