The tech billionaire is in the process of building a luxury estate, rumored to be costing him a cool $260 million, but included in the plans is a secretive underground bunker that's twice as big as an average Australian home.

Reports about the project sparked concern in certain corners of the internet and saw a flood of inquiries directed at companies that construct sub-terrain shelters.

Zuckerberg, executive chairman and CEO of Meta, which owns Facebook, Instagram and WhatsApp, hasn't commented on the bunker revelations and anyone working on his property is bound by a strict gag order.

But it appears he has been quietly planning his fortified retreat for at least a decade.

Zuck's shopping spree

It was Christmas in 2016 when Zuckerberg took to his social media platform to share happy snaps of his family enjoying the Hawaiian island of Kauai.

Home to about 73,000 locals, the pristine piece of paradise – known as 'The Garden Isle' – has served as the setting of major Hollywood productions, including Pirates of the Caribbean and Jurassic Park.

Most residents are descendants of Native Hawaiians, as well as Chinese, Puerto Rican and Filipino migrants who came to work on sugar plantations in the late 19th Century.

It's also beloved by tourists from around the world, and more recently by the rich and famous.

"A few years ago, Priscilla and I visited Kauai and fell in love with the community and the cloudy green mountains," Zuckerberg wrote in 2016.

"We kept coming back with family and friends, and eventually decided to plant roots and join the community ourselves.

"We bought land and we're dedicated to preserving its natural beauty. It's filled with wildlife like pigs, turtles, rare birds and seals, and local farmers use it to grow fruits and spices. I love taking Max to explore and see all the animals."