The search for British TV doctor Michael Mosley, who went missing Wednesday on the Greek Island of Symi, has reportedly been narrowed down to a cave ominously known by locals as “The Abyss.”

Mosley’s wife, Clare Bailey, began ringing the alarm bells Wednesday evening after Mosley didn’t return at 7:30 p.m. from a walk he had departed on six hours prior.

Authorities began searching for Mosley that evening, believing he may have fallen on his walk. “Nothing at this stage can be ruled out,” a police source told The Daily Mail, Thursday. Mosley was last seen on a Saint Nikolas Beach at 1:30 p.m., according to a user in a Symi Facebook group who posted a photo of him. Authorities and search parties scoured the coast but found no sign of him.

Authorities now believe Mosley disappeared into the mountains, which harbor a vast network of dangerous caves. “It’s like a deep system of tunnels, but it is full of water and can spread for kilometers. There is a reason they call it ‘The Abyss.’ There is something very strange going on,” a waiter on the Island told The Telegraph.

Despite the blistering heat, firefighters are reportedly searching the cave system for Mosley. Nikitas Gryllis, the deputy mayor of Symi, told local media the mountainous area where Mosley vanished is especially tricky to search.

“It is a strange area, a mountain with many blind and dangerous spots, which must be combed meter by meter,” Gryllis said. Rescue workers reportedly fear that Mosley “took a wrong turn” somewhere, with one saying it is now “a race against time” to save him. Mosley’s wife, meanwhile, has vowed not to “lose hope” and said the last three days have been “the longest and most unbearable days” for the whole family.