M24 Chaffee Light Tank

[Bruce_in_Oz]

The reason for the scale and longevity of the Cu Chi tunnels is a combination of two local features.

Firstly, the soil is a weird, hard, sandy clay stuff that needs almost no shoring or props.

The other factor is a VERY deep water table. Nearby waterways run in the same, concrete-like soil and did not seem to cause too much trouble. There were rumours / propaganda that the VC dug right under major streams and rivers, but I would take that with a measure of salt.

The area is in a part of the world subject to monsoonal downpours in the "wet" season. As in the mining biz, flooding is up there with "cave-in" as something you don't want when you are crawling on your guts, in the dark, fifty feet below the surface.