Verticality is an important part of this level's design: there are a number of large halls with ledges and elevators where enemies are placed above or below you. Those enemies consist of Pfhor Fighters, Troopers and S'pht Compilers.
There are three pattern buffers in this level. One can be found in a room to the southeast of the start location along with the Fusion Pistol, one can be found in a room in the north, and the last one is located on a hard to reach ledge in one of the south halls. That ledge also contains a 2x shield recharger. Another 2x shield recharger is found at the end of one of the corridors surrounding the starting room.
The first room of the level is designed to teach you a trick: buttons can be triggered by grenades. So, switch to your Assault Rifle and use the grenade launcher to hit the four switches through the small windows in the walls. There's another corridor visible up above that leads to the exit of the level, but it's currently inaccessible.
Go to the south corridor, then head west. You'll find a Fusion Pistol as well as a pattern buffer here.
Return to the starting room, then head east. There's a 2x shield recharger here. Now make your way north, kill the enemies you find and get up on the ledge using the elevator in the northwest corner. Move along the ledge (you can find a pattern buffer here) and drop down into the next room. The wall across from you drops down, revealing that it's actually a large elevator platform filled with enemies.
After dealing with the enemies, go up the elevator. The corridor on you left hand side leads back to the starting room, where you can recharge your shields if you have to. Once you're ready, drop down from the ledge. The walls will open up, revealing groups of enemies.
One of the two buttons here activates the north elevator back to where you came from, the other one activates the south elevator. Get on the south elevator to move on.
The next room has a series of platforms that move in a wave pattern once you stand in the middle of the room. If you stand on the platforms too long, you'll be crushed against the ceiling, so move quick. You can use the first platform to use the ledge with a 2x shield recharger and a pattern buffer, but getting up there is a bit tricky so make sure you have enough health. The last platform will take you to the opening in the west wall.
In the next room to the west, a switch raises the large elevator in the middle of the room; you can use this later to retrace your steps. First, move north, descend the stairs, and hit the two switches at the end of the corridor. Now go all the way back to the starting room of the level, where you will find an elevator has been activated.
Use the elevator to jump to the corridor up high on the southeast wall. You'll soon reach a room with lots of invisible Compilers. The exit terminal is located just north of here; it will take you to G4 Sunbathing.
Leela tells you that she wants to send a transmission to Earth warning them of the Pfhor, but the UESC Marathon's long range transmitter array is down and you must reset the secondary transmitter (this is the objective of the next level). Before sending you out into a vacuum however, she wants you to collect the Zeus Class Fusion Pistol, as your current weaponry is ill-suited for vacuum.
- The music for this level is titled Flowers In Heaven. It is track #4.
- The named regions on the level map include "Experimental Vacuum Chamber", "Charge Conduit (North)", "The Wave", and "Lift #A7".
- When this map is opened in a map editor, a written message can be found outside the bounds of the level. It reads, "poenas dare".
- The level's name refers to the Zeus Class Fusion Pistol that is found here.
- Cold Fusion is an actual science term. Cold fusion is a hypothetical type of nuclear reaction that would occur at, or near, room temperature.
- This is the first level where 2x shield rechargers are found.
Marathon - Level 7: Cool Fusion