Agartha is a mythical city featured in the Uncharted comic miniseries.
Agartha is an ancient, underground lost city that resides near the center of the earth. It was found by explorer Sir Richard Byrd during one of his expedition flights over the North Pole. Because of his discovery, Byrd was one of the first to propose the Hollow Earth theory. Agartha was the home to an underground civilization that was eradicated by creatures that lived in the earth called the Smoking Gods. The city's people used underground flying machines for transportation, and the entire city was lit by a wall of diamonds that were holding back a flow of lava (what Byrd called the Second Sun).
Events in Uncharted comic series
Michael and Rose Doughty were searching for the Amber Room (which lied somewhere in Agartha) to restore their family name, but Nathan Drake, Victor Sullivan, and Chloe Frazer also planned to find the Amber Room before they did. On the back of the Amber Seal were coordinates that led them to the Amber Room outside the Gates of Agartha. The Doughtys forced Sully to fly a plane down into a gorge a the North Pole and landed in a cavern where the gates were at.
All though the Amber Room was found destroyed by the Smoking Gods, Drake and Chloe entered through the main entrance leading to Agartha. They spotted paintings on the walls by Richard Byrd, warning them that the gates were meant to stay closed so the Smoking Gods wouldn't be released upon the world. After navigating through dark tunnels and chased by several Smoking Gods, Drake, Chloe, and the Doughtys reached Agartha.
As the group explored the city, Chloe took a picture of one of the flying machines for her client. Rose, amazed by the wall of diamonds lighting up the city (the Second Sun), began to pull one of the daimonds out of the wall. She shot her brother dead after he protested with her, and pulled a diamond out of the wall, releasing a flow of lava upon the city which killed Rose and destroyed the entire city. Drake and Chloe escaped the crumbling city as it was engulfed by the lava, lost forever.