The tunnel rats were American, Australian, New Zealander, and South Vietnamese soldiers who performed underground search and destroy missions during the Vietnam War.

During the Vietnam War, "tunnel rat" became an unofficial specialty for volunteer combat engineers and infantrymen from the Australian Army and the U.S. Army who cleared and destroyed enemy tunnel complexes. Their motto was the tongue-in-cheek Latin phrase Non Gratus Anus Rodentum ("not worth a rat's ass").

https://en.wikipedia.org/wiki/Tunnel_rat