Tunnel 2 : A covered excavation used for the conveyance of people or materials, typically no smaller than 0. 91 m (36 in). in diameter and within 20 degrees of horizontal
NFPA 1670 Standard on Operations and Training for Technical Search and Rescue Incidents, 2009 ed. https://www.nfpa.org/assets/files/AboutTheCodes/16 70/Errata1670-09-1.pdf, (Std: Opr and Trng for Tech Search and Rsq Incid - Amer) {408 NFC}