Google is celebrating the discovery of the Sơn Đoòng Cave cave in its new Google Doodle.
The once-forgotten cave was rediscovered in 2009 and contains amazing natural wonders and even a community of monkeys contained within its staggering, jagged walls of limestone rock.
Google often marks an incredible discovery, the achievements of significant people or the world's natural wonders with a special Google Doodle on appropriate days.
The Vietnam natural wonder is believed to be between two and five million years old and its name means "Mountain River" in the local language.
So what is the cave, and how was it discovered?
What is the Google Doodle?
The new Google Doodle shows the Hang Son Doong cave, found in the deep jungle in a remote area of Vietnam
Google said: "Today’s Doodle celebrates this colossal cavern, which was officially discovered on this day in 2009."
Google say the cave was "officially discovered" on this day because it is believed that it was actually stumbled upon by local farmer Ho Khanh in 1990.
A description of his initial discovery on Oxalis Adventure reads: "Local man Ho Khanh discovered Son Doong in 1990 while searching Phong Nha-Ke Bang National Park for food and timber to earn a modest income.
"During his search he stumbled across an opening in a limestone cliff, noticed clouds billowing out from the entrance and heard the sounds of a river raging from somewhere inside. He returned home and forgot about the cave."
He is said to have retraced his steps in 2008, finding the cave once again, before Howard and Deb Limbert of the British Cave Research Association documented the world wonder a year later.
The cave contains incredible wonders, but travel there is limited and can only be visited on four-day expeditions with a specialised travel company.
What is the Hang Son Doong cave?
The Hang Son Doong cave in Vietnam is considered to be the world's largest cave.
This is based on a length of around 5.6 miles and has a volume of 38.5 million cubic metres. It was recognised by the Guinness Book of World Records as the largest cave in the world in 2013.
The cave is a geological wonder and is said to have the world's only underground living monkeys, along with flying fox bats.
Its enormous depths are reported to have a rainforest with its own weather ecosystem, with giant sinkholes allowing sun and rai to cascade down into the cave.
Other discoveries in the deep depths of the cautious cave include the world's largest limestone pearls and the world's tallest stalagmite around 70 metres tall.
The whopping height of the stalagmite doesn't compare to the overall height of the cave, stretching to 200 meters, or around the same height as the HSBC tower in Canary Wharf, London.
Within the cave, the main cavern is "large enough to house an entire New York City block".
Located within Vietnam's Phong Nha-Ke Bang National Park, plans to build a cable car for mass tourism were rumoured to be taking shape, but were later dismissed by Vietnamese authorities.
