Tech billionaire Mark Zuckerberg is expanding his extensive compound on Hawaii’s Kauaʻi island – including an extensive bunker – in a secretive building project. 

Zuckerberg has aggressively bought up land on Kauaʻi and owns more than 810 hectares dubbed Ko’olau Ranch, which is complete with two mansions and an extensive so-called “doomsday bunker”.

Earlier this year, the Meta CEO quietly made another purchase of about 340 hectares close to add to his holdings.

Guthrie Scrimgeour, a former reporter for Hawaii’s The Garden Island newspaper, claims Zuckerberg is expanding his compound on a burial site in a secretive project.

“What drew me to this story was just the fact that there seemed to be on Kauaʻi, hundreds of people that were working on the Zuckerberg property, but who weren’t allowed to talk about it, because everybody that works there has to sign a pretty strictly enforced [non-disclosure agreement],” Scrimgeour told Hawaii Public Radio.

Scrimgeour said he filed a public records request to try to find out what Zuckerberg was building.

“There are … I would say, dozens of structures on that land centred around these two massive mansions that each have a total square footage comparable to a NFL football field,” he said.

“There’s a massive gym. There’s multiple pools, hot tubs, cold plunges, sauna, these weird saucer-shaped tree houses.”

Scrimgeour said the buildings also included a bunker between the two main mansions.

“There is a tunnel that branches off into this 5000-square-foot (465-squaresmetre) storm shelter, which has a blast-resistant door and an escape hatch,” he said.

Satellite images of the property also show dozens of buildings that have not yet appeared in public records requests.

Documents recently released through public records show plans for three more large buildings, ranging from 726- to 1036- square metres.

Two of the buildings have 16 bedrooms and 16 bathrooms between them, with a shared verandah spanning more than 120 square metres.

The buildings have cameras, keypad locks and motion detection devices.

Zuckerberg spokesperson Brandi Hoffine Barr described the buildings as short-term guest housing for family, friends and staff.

Scrimgeour wrote in Wired Magazine that – based on counting bedrooms alone that he’d seen in the planning documents – he estimated the property could comfortably house more than 100 people when complete.

“One thing I found interesting … is just that the total spending on Zuckerberg’s compound, which we estimated is probably over US$300 million ($457 million) by now,” he said.

“That is probably more than the Kaua’i operating budget for fiscal year 2024.”

A local islander who fished in the area contacted Zuckerberg’s representatives about 10 years ago to inform them that part of the compound housed the remains of his great-grandmother and her brother, according to Wired.

Julian Ako negotiated with Zuckerberg management for months before finally being able to gain access to the burial site and register the graves with Hawaii’s Department of Land and Natural Resources.

The burial site, first identified in 2015, was “fenced off and maintained” after being discovered, Zuckerberg spokesperson Hoffine Barr said.

“Mark and Priscilla continue to make a home for their family and grow their ranching, farming, and conservation efforts at Ko’olau  Ranch,” Hoffine Barr said of the Zuckerbergs and their Hawaiian property.

“The vast majority of the land is dedicated to agriculture – including cattle ranching, organic ginger, macadamia nut, and turmeric farming, native plant restoration, and endangered species protection.

“After purchasing the ranch, they cancelled the previous owner’s plans for 80 luxury homes.”

Zuckerberg began buying land on Kauai in 2014, initially acquiring 280 hectares near the town of Kilauea.