The Abandoned Lab is an area accessible from the Antennae Field.
This area is a big, subterranean laboratory with many doors and pools full of toxic residuals, other points of interest include various research facilities, mutant-like NPCs, most of them swimming in the toxic pools, and also a large room full of imprisoned robotic Plague Nurses, one of them gives you the Metallic Alter.
Coming from the Antennae Field, you'll be right next to a set of stairs going down to a closed metallic door, opening it will take Mariana to a short corridor. Going through the corridor and taking the lower left door will take you to the room with the Robotic Plgue Nurses, interact with the one whose cage is open to get the Metallic Alter and exit through the door to the right, and from there, take the door to the upper right section to proceed to the next room.
From this room, going all the way up will eventually take you to a more open section with a series of stairs going upwards. Taking the first stair and going to the one at the left (Right next to the Half Body Scientist) will lead to another room with a single door inside. Entering it will take Mariana to the Dark Library.
If you go to the right instead and then climb through the ladder will take you to the highest section of the room, opening the door with the green light will lead to another corridor. Going through the corridor and taking the right path at the intersection and then to the lower left section will take you to a small room with only a corridor going downwards. Interacting with the cubic figure at the end of the corridor will take Mariana to the Puppet Path.