to warn you before you start playing the cut PC version ... this game was a Xbox exclusive till the very end ... the PC/PS3 version has only 1/4 of the original game that you got on XBox360 ... not only it missed maps , has awful plot holes due those cut levels and a awful ending ... compared to the Xbox 360 version that has the missing 3/4 of the game ... on PC/PS3 this game was a disaster compared to the Xbox 360 version ...
if you ask what is missing : 7 maps from the original game that point to Beneath the Ashes (the underground catacombs under Lara mansion) and Lara's Shadow (when you play as the Atlantis clone) ... and the ending was cut short ... in other words 10h of TR underworld was cut ...
PC\PS3 ending
https://youtu.be/B1S3BsuQ2Mo?t=717Xbox360 Ending
https://youtu.be/SqBy9wxzHZU?t=574