If you are in need of supplies, purchase them from the Circus of Values vending station to the right of the door. It doesn't matter which way you turn here, they both lead to the same place. You can find another Circus of Values vending station, as well as a Gene Bank within the next area. Take the audio diary (Ryan Takes F Futuristics) in the crate next to the Circus of Values machine. There is also a Turret around the corner, be careful. Proceed through the door below the "Office of Andrew Ryan" sign. You'll find yet another Circus of Values machine in this area. An audio diary (Scoping The Gate) can be found on the ground near the machine.
Approach the table in the middle of this area and pick up the Grenade Launcher on it. You can also find a U-Invent station in this area, it is right by the next door that you need to go through.
The Office of Andrew Ryan
Moving on, in the next room, search the first corpse on the right for an audio diary (Stopping Ryan). The fourth corpse on the right also contains an audio diary (Going To Heat Loss). Once you have these two diaries, a large group of Splicers attack you. Now, you are probably already aware of the fact that Splicers become stronger with each subsequent level. However, this only really becomes noticeable once you reach Hephaestus. Hopefully, you have been researching all forms of Splicer during your encounters with them in previous levels. The research bonuses gained from researching your enemies are a great help. Before you leave, go up the stairs to the right of the Andrew Ryan bust and enter the room there. On the desk inside of this room, you'll find an audio diary (A Man Or A Parasite) and the "Frozen Field 2" tonic.
Frozen Field 2
Use the Circuit Breaker in front of the door to Ryan's office, then go through the door marked "Hephaestus Core" on the opposite end of the room. Through the next door, travel down this path until you reach the next area. You will more than likely encounter the first Big Daddy of the level here. If you have been rescuing all of the Little Sisters that you have encountered up to now, Tenenbaum will send you another gift. Next to the chair at the end of the walkway here, pick up the audio diary (Fontaine's Legacy).
To Hephaestus Core
Heat Loss Monitoring
Down the stairs, move along the walkway here until you reach the Gatherer's Garden station. An audio diary (Assassin) can be found on the ground next to this machine. There's also a "Power To The People" Weapon Upgrade station on the wall across from the Gatherer's Garden. Down the stairs, enter "Heat Loss Monitoring." Don't bother with the Circus of Values machine on the left, it only dispenses live bombs. Ha, what a riot.
Power to the People
In Heat Loss Monitoring, the lights flicker on and off a couple of times. At the opposite end of the area, a couple of seemingly dead Splicers spring to life and attack you. You should encounter another Big Daddy in the proceeding area. As always, make sure he has a Little Sister with him before you attack. There's an audio diary (Kyburz Door Code) in the far right corner of this room. Take the stairs down to the lower level of Heat Loss Monitoring. At the bottom of the stairs, turn left. There is an audio diary (Genetic Arms Race) on the desk to the left of the generator in the flooded area. You will take damage if you are standing in the water, so you have to be quick.
Take the stairs straight across from this area down to Pablo's workspace. The “Security Evasion 2" tonic and the audio diary (Getting A Break) that you are looking for are both on the desk. Pick up the audio diary for a new goal.
Audio Diary and Tonic
Backtrack to the main Hephaestus Core area, turn right and go down the stairs. There is an audio diary (Impossible Anywhere Else) and some ammo on the desk down there. Back up the stairs next to the Heat Loss Monitoring door, there is an audio diary (Running Short On R-43s) that can be found on top of an overturned filing cabinet to your left. The "Workshops" door is right across from this area. The last Big Daddy (if you've been following along) can be found in this area. Approach the Ammo Bandito vending machine, then head down the stairs across from it. At the bottom of the stairs, enter the office in front of you and take the audio diary (Device Almost Finished) that's on the desk.
There is a Turret on each side of the door to Kyburz's office, deal with them first. If you recall, during the "Kyburz Door Code" audio diary, Pablo Navarro explained that the code to Kyburz's office was set to the date of Australia Day. Australia day is on January 26th, so the code to unlock the door is 0126. The room is full of traps, be extra cautious as you navigate to avoid taking damage. An audio diary (Great Chain Moves Slowly) can be found on Kyburz's desk. Another Weapon Upgrade station is on the wall to the left of Kyburz's desk. Leave Kyburz's office and turn left. In the next room, search the corpse laying on the floor to find an audio diary (The Dream).
You'll find a crawlspace door at the very end of this area. Open it and crawl inside. The table to the left holds an audio diary (Assembling The Bomb) and the Damage Research tonic.
Finishing Kyburz's Bomb
Most of the materials that are required to finish assembling Kyburz's bomb can be found in the Workshops area. You need two quarter-cans of Ionic Gel, some Nitroglycerin and four R-34 Wire Clusters. The R-34 Wire Clusters are found on Elite Bouncer corpses, you'll see one of them once you head back through the crawlspace. The Nitroglycerin is found in Kyburz's office. There is a switch on the side of the desk at the back of his office, pressing the switch opens the grate covering the Nitroglycerin on the wall next to the desk. A Turret also appears at the front of the office.
R-34 Wire Cluster
There is an audio diary (Market Maintenance Code) underneath the stairs near Kyburz's office. To get underneath these stairs, use the crawlspace door on the wall to the left at the bottom of the stairs leading up to Kyburz's office. The remaining R-34 Wire Clusters that you need can be found on the bodies of the Bouncers that you have killed in order to harvest or rescue the Little Sisters. For this, you will have to backtrack to the main Hephaestus Core area. Before you do, though, one of the Ionic Gel cans that you need can be found at the end of the upper Workshops area. Back in the main Hephaestus Core area, turn right. Move in this direction and go down the stairs that you come to. The second can of Ionic Gel that you need can be found on top of the machine along the wall at the bottom of the stairs.
For the remaining R-34 Wire Clusters that you need, if you have to, kill the re-spawned Bouncers in the previous areas. When you have all of the materials, return to the EMP Bomb Casing. Place all of the materials inside, then pick up the EMP Bomb.
Overloading The Core
With the EMP Bomb in your possession, return to Hephaestus Core and locate the door to "Geothermal Control." Go through the door and move through the first area. There are two Turrets in the next room. Hack these two Turrets to get them on your side. Clear the room of any Splicers, then use the Magma Release Valve to redirect the magma flow. While you hold the A button to turn the valve, Splicers will enter the room from both sides and attack you. If you hacked the two Turrets in this room, they should be able to keep the Splicers at bay. However, if need be, cease spinning of the valve and deal with any Splicers that have managed to get too close to you.
Magma Release Valve
Once the magma flow has been redirected, turn around and use the Elevator Button to call the elevator down to your level. On your way to the Core, pick up the "Shorten Alarms 2" tonic that is on the ground by the corpse just before the lift. Place the EMP bomb on the core.
Leave Hephaestus Core and return to the room before Andrew Ryan's office. Use the Circuit Breaker in front of the door to Ryan's office, this causes the mag-locks preventing your entrance to overload and explode. Proceed through the Bulkhead to Rapture Central Control. Move forward, look for the audio diary (The Vita-Chamber) on the desk next to the Vita-Chamber to you left. At the end of this area, take the flight of stairs on the left up to the top. Go through the vent and drop down into the area at its end. Pick up the two audio diaries (Mind Control Test and Baby Status) that are on the table straight in front of you.
Go through the door here. In this room, watch the events that unfold. After, take the key and use it on the Self Destruct Override in the next room. After the transmission, follow the Little Sister to complete the level.