Upon entering the lower levels, you'll encounter a heavily flooded corridor, making direct passage impossible. To bypass this, you need to find a way to drain the water. Look for a maintenance shaft entrance to your left, partially obscured by debris. Crouch and enter the shaft. Follow the narrow passage until you reach a small control room. Inside, you'll find a large lever and a pressure gauge. The lever controls the water pump. Interact with the lever to activate the pump. The gauge will slowly rise, indicating the water level is receding. Wait until the gauge stabilizes at 'Empty' before exiting the control room. Return to the corridor; the water will now be gone, revealing a path forward and potentially new threats.
