An intrepid British explorer who went on an expedition to find a reclusive tribe in Papua New Guinea has disappeared, his worried family said this week.

Benedict Allen, 57, was dropped into the rainforest via helicopter last month and was expected to return to Papua New Guinea’s capital Sunday, BBC reported. His sister, Katie Pestille, expected to hear from him Monday and said he missed his flights home.

“It’s ghastly,” she told BBC Radio 4. “For everybody else it’s very exciting — all the expeditions and all the things he does, but for his sister and his wife, it’s more of a worry.”

As of Wednesday, the pilot who dropped Allen off, had started searching for him, starting in a remote area called Bisoria, according to the outlet. Local police chiefs were also trying to locate him via helicopter and get him out.

The explorer was traveling solo and wasn’t carrying a satellite phone or any GPS devices.

Allen was looking for the Yaifo, a tribe in a remote mountainous area he met in the 1980s in the hopes of making a documentary about them.

“No outsider has made the journey to visit them since the rather perilous journey I made as a young man three decades ago,” Allen wrote in the last entry on his blog. “This would make them the remotest people in Papua New Guinea, and one of the last people on the entire planet who are out-of-contact with our interconnected world.”

When he met the Yaifo on his first trip they initially greeted him with suspicion and hostility but then accepted him, he said, adding that he was unsure how they would receive him this time.

He also warned that he may be unreachable during his treacherous journey.

“Don’t bother to call or text! Just like the good old days, I won’t be taking a sat phone, GPS or companion. Or anything else much,” he wrote on his blog. “Because this is how I do my journeys of exploration. I grow older but no wiser, it seems …”

In an Oct. 11 tweet, he posted a picture of himself heading off to Papua New Guinea, “Don’t try to rescue me, please,” he wrote.

The father of three has filmed a number of his adventures for BBC documentaries and is also an author.

He previously crossed the Amazon Basin on foot and in a dug-out canoe, participated in a six-week male initiation ceremony where crocodile marks were carved onto his body and once ate his own dog to survive.

He told Lonely Planet that his one travel tip is to always keep toilet paper in a back pocket.

“For me personally, exploration isn’t about conquering nature, planting flags or leaving your mark. It’s about the opposite: opening yourself up and allowing the place to leave its mark on you,” he once told the BCC.