The City's Secret is the eighth chapter of Uncharted 2: Among Thieves.
Plot
After battling through several soldiers, the four came across the entrance to the temple. Elena asked Nate what he was after. He pointed her to the symbol of the Cintamani Stone located on the building, and explained to her that it was a massive raw sapphire worth millions. Chloe then pulled Nate away and soon after, they spotted another chopper and quickly headed inside.
Once inside, Elena tried to convince Nate that Lazarevic was after something other than the money, but was quickly pulled away by Chloe, who was intent on getting what they came for. He and Chloe went ahead and searched the temple, leaving Elena and Jeff at the door.
Heading further into the temple, the two caught the sight of a massive statue, holding makeshift Cintamani Stone. Upon examining the structure, Nate realized what it was . He took out his diary and figured if he could get the arms into the right position, they would unlock a secret entrance withing the temple. After doing some skillful climbing, Nate managed to adjust the arms to their proper position. This caused the fake Cintamani Stone in the statue's hands to open up, revealing a key tumbler that fit the size of the dagger. Nate then pulled out the dagger and realized it was also a key. Nate ascended to the key hole and activated the tumblers, causing the water in the pool to drain, thus revealing a small, narrow entrance leading underground.
Nate and Chloe searched through the underground area. Along the way, they spotted a wall with a symbol looking somewhat like a tree, and wondered what it's connection to the Cintamani Stone was. As they progressed further, they fell through a trap door and were met with a deadly surprise - spikes! Nate acted as quickly as he could and shot the gears controlling the contraption, which stopped the spikes from ultimately crushing them. The two quickly left the area and continued navigating the temple. Shortly after, they stumbled upon a humongous room full of mirrors and a very important structure staggering above them - the Phurba dagger.
Walkthrough
As the chapter begins head into the chamber where the giant multi-armed statue is. To proceed to the next area the player has to position the statue's arms to match the diagram Nate has in his notebook.
After climbing onto the broken pillar to the right side of the room, the player is facing the statue. After heading right and using the walkway to reach the statue's right arms, the player moves across the gap by swinging from the wooden posts overhead. The hand holding the skulls is lowered and followed by the player climbing up to reach the upper arm (holding the dagger). Jumping onto the arm knocks it down into position.
Once the arms are in place, the blue stone that the statue holds breaks apart revealing a spot for the Phurba Dagger to be inserted. The use of the dagger lowers the water in the nearby pool and the player must enter the passageway that has been revealed. To proceed, the player descends down the corridors..
As the player explores the corridor, the floor drops out from underneath and the player falls into a room where the ceiling descends with numerous spikes pointing down. The player must quickly shoot out the wooden notches from the cogs on the room's wall to stop the ceiling from crushing and impaling the player.
With the ceiling no longer descending, climb out and head down the hallway to end the chapter.