First you have to go over to lara's diving board next to her swimming pool. push the button on the back of it. This will open up a secret door in the main room. go over to it and there will be a switch or lever. push the button or lever and another secret door will open.

Now here you have to hurry up. Press O and then sprint once you are about half way across the room. at the end do a forward role. now you are in her secret room. It has the artifacts from the other tomb raider games plus the t-rex head above the fire place.