Trajan's Markets - Secret #2 and Valve Wheel
After destroying the mechanical head and retrieving the MARS SYMBOL, as shown in the previous section, enter the small side room with the RED MACHINE. (You will already be here if you used the alternate method for destroying the head.)
The machine has an empty stem where a wheel would fit, but you don't have one yet. Climb onto either of the blocks flanking the doorway. Walk to the edge nearest the machine. Then jump straight up and grab the metal structure overhead. Pull up. (It's dark in here, so light a flare if necessary.)
Then jump onto the machine. Here you'll find a golden rose (19) (52/239) (SECRET #2, 5/36). Drop down and return to the room where the head was.
On the opposite side of the room, below and to the left of the entrance, pick up some revolver bullets (20) (53/239).
Then stand at the edge of the nearby manhole cover, on the side of the circle with the brown wedge, and press Action to open it.
Drop through the hole...
...and follow the short passage to a T-shaped pool. One of the semi-circular grates (the one in the far right corner from the entrance) releases a swarm of rats when Lara walks past, but you can avoid it easily. If you're going for the "Tail Innocence" achievement/trophy, you do not have to spawn the rats and outrun them. Just avoid that ledge completely.
If you accidentally trigger them, jump in the water and swim toward the bottom. The rats stay on the surface so they can't bite Lara. They should disperse before you run out of air.
At the junction of the "T" there's an underwater fan. It's not dangerous, but it does prevent you from swimming to the bottom of the vertical shaft. Swim into the wide, rectangular alcove just above the vertical shaft and pick up a small medipack (21) (54/239).
Return to the junction. Note the two openings on the side walls of the shaft that leads down to the fan. The current prevents you from reaching the lower opening. Take a breath. Then swim through the upper opening.
Follow the short but twisty passage to a SMALL ROOM WITH A PIPE AND LIT TORCH. Grab some shotgun shells (22) (55/239) and use your CROWBAR to pry the VALVE WHEEL (23) (56/239) off the pipe.
Swim back to the T-shaped pool. Climb out of the water and return through the short passageway to the ladder.
Climb through the open manhole to get back to the room where you destroyed the mechanical head. Then cross the lower level into the the ROOM WITH THE RED MACHINE.
Press Action to place the VALVE WHEEL on the stem protruding from the red machine. Press Action again to turn the wheel. There's no cutscene, but this turns off the fan back in the SEWERS.
Return through the manhole to the SEWERS.
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