Find the Dark One in the Gardens | Chapter 1: Sparta Metro: LL Guide

Follow Anna, choosing a new exit from the conference room. Stop in the next room and take a hidden Artyom's diary page shown on the above screen (collectible). It is worth to open a diary and read a new entry.

Now you can enter the elevator. After getting to the new level, wait until train appears. Enter the wagon, thus ending the first game chapter.