Inside Menkaure's Pyramid - Stairs Down to Room with Stone Basin

Start down the stairs.

Partway down, Lara will look up at a recessed spot in the ceiling containing a gold star.

Use the revolver or crossbow combined with the laser sight to target the star. One shot should break it, opening a trapdoor in the room below.

Continue down the stairs a little farther and you'll see 2 mummies shambling around near a white marble basin. If you wait for them to move near each other...

...you can destroy both mummies with a single grenade or explosive arrow. (Other weapons are ineffective.)

Switch to pistols, continue to the bottom of the stairs and kill another bat.

Pick up the revolver ammo on the floor behind the basin.

Then jump into the basin...

...and drop through the trapdoor you opened by shooting the star.

[Inside Menkaure's Pyramid Walkthrough]