Cu Chi Tunnels

Initially used in the war of independence against the French, the Cu Chi tunnels near Saigon were well established by the time the US dropped in. In fact the US 25th Infantry Division (Tropic Lightning) established their base right smack on top of an enormous complex.

Everything was connected underground.

Even this "Spider hole."

The tunnels were dug into and through laterite clay, which sets so well it looks like concrete.

Even B-52 drops couldn't penetrate them.

And then I got to unload a banana clip of AK-47, semi-git 'em and auto-git 'em. At two bucks a round it was a bargain.