After using the SECOND CANOPIC JAR and walking across the water in the BLUE-LIT ROOM WITH THE POOL AND SEATED STATUE, as described in the main walkthrough, enter the passageway behind the statue, follow it to the end, and climb up on the right side.

Approach the doorway and drop carefully onto a block overlooking a crocodile-infested pool.

There are 2 crocodiles swimming around when you arrive. They can't reach Lara on the block so save ammo and shoot them with pistols.

Now turn so Lara's back is toward the pool, drop back and hang from the edge of the block, then immediately pull up. This draws out a third crocodile, which you can also shoot from above.

When the coast is clear, pick up some shotgun shells on the sand near the west wall...

...and more shotgun ammo on the bottom of the pool. Swim down through the opening beneath the obelisks.

Follow the short passageway around to the right to an opening where you can surface.

Climb out of the water and press the button.

Doing so lowers the cage between the obelisks, revealing a silver key and golden statue sitting on a pedestal.

Swim back out to the pool the way you came. Climb out of the water on either side of the obelisks and go behind them...

...to climb up near the pedestal. Press Action twice to take the HYPOSTYLE KEY and the SUN GODDESS from the pedestal. These are two separate pickups. Make sure to get both items.

[Temple of Karnak Walkthrough]