Their expedition is the subject of Thursday’s “Oregon Field Guide” telecast. OPB’s outdoor show will kick off its 26th season with “Discovery on Mount St. Helens.”

The volcano literally is a forbidden landscape. People need permits to gain access. Ten experienced mountaineers, several affiliated with the National Speleological Society, had that access during a five-day stay in the volcano’s crater; Jahn and OPB videographer Todd Sonflieth were allowed to document the visit.

“We have lots of criteria for going in — and into the crater, in particular,” said Peter Frenzen, Mount St. Helens National Volcanic Monument scientist. “We look carefully at what the objectives are. Does it provide new knowledge?”

As far as officials knew, no one had mapped and documented these glacier caves, Frenzen said. “It’s an unseen feature, and to have Ed and ‘Oregon Field Guide’ along allows the public to see something that no one would otherwise be able to see. We do like to have folks see what is out there in nature.”

The mountaineers looked at two cave systems within the crater that are about a quarter-mile apart.