Fifth Ark

The Fifth Ark (フィフス・アーク, Fifusu Āku) is a location in Final Fantasy XIII. It is an old decrepit building the party crashes into after escaping the Palamecia. It is where the party fights Cid Raines in a boss battle, and Fang summons Bahamut for the first time. The party also departs for Gran Pulse from here through a special gate. The area contains tunnels with remnants of trains like present-day subways, and modern-day skyscrapers similar to those in the Yaschas Massif.

Datalog
This Gran Pulse structure lies deep below the capital city of Eden, its existence unknown to the members of the Sanctum.

''According to legend, Arks are Gran Pulse armories that hold living weapons in stasis, in preparation for war with outside forces. These facilities also supposedly serve the function of awakening a l'Cie's latent powers.''

Why this 'Fifth Ark' is on Cocoon, and whether there are other Arks hidden elsewhere, is uncertain.

Story
After the party escapes from the Palamecia, they are flown into a building below Eden that houses the Fifth Ark, a facility created by the Pulse fal'Cie. The party soon begins to question if they should complete their true Focus to destroy Cocoon, or if should they carry out Serah's wish to save it.

According to Fang and Vanille, there was a legend on Pulse that spoke about Arks, or facilities the fal'Cie created to build weapons of war. Many had tried to look for the Arks but were unsuccessful. The Arks were used as training grounds of a sort to make the l'Cie stronger.

While exploring the Ark, the l'Cie encounter Cid Raines and discover he is a Cocoon l'Cie. Branded by the fal'Cie Barthandelus, Raines's Focus is to aid the party's destruction of Cocoon under the fal'Cie's guidance. Raines tells the l'Cie the real truth about the fal'Cie: the fal'Cie wish to destroy millions of lives on Cocoon so that "The Maker" would return to the world. The fal'Cie, unable to carry out this goal themselves, brand humans and turn them into l'Cie, all while tending to the Cocoon populace. Cid attacks the l'Cie in an attempt to foil the fal'Cie's plan, believing that if he kills them, it will not be possible for them to destroy Cocoon. Cid is defeated and turned to crystal, despite having failed his Focus.

Snow then decides he should defy his Focus like Cid did and try to save Cocoon, even if it might mean he will turn into a Cie'th. Voicing his views to the team, they all side with him apart from Fang, who grows furious because she does not want to see them all turn into Cie'th. Resolving to go on alone, she turns her weapon on the party, but in her rage, she unwillingly summons her Eidolon Bahamut. Lightning and Vanille help her defeat the beast.

After the battle Fang realizes the Eidolons appear whenever a l'Cie begins to waver on their path towards completing their Focus, forcing the l'Cie to choose life or death. At the lowest level, the party discovers an airship and a portal to Gran Pulse. Realizing that it is their only way out of the Ark they board the ship and fly through the portal and into the skies above Pulse. The airship is destroyed following an attack by one of the local wildlife, sending the team flying out of the ship. Fang saves everyone by summoning Bahamut and they ride it down to the world below.

Later, during the l'Cie's assault on Eden, many species of Pulsian wildlife (including Behemoths, Adamantoises and Amphisbaenas) invade the area via summon gates due to (what is speculated as) the fal'Cie activating the Fifth Ark below Eden. These are presumably all killed during both the battle and Ragnarok's final transformation.

Enemy Formations
In addition to the monsters listed below, two Phosphoric Ooze can merge into one Alchemic Ooze, an Imp can summon an Ahriman and a Berserker can forge a Centaurion Blade. None of these enemies award additional CP.

Vestibular Hold

 * Pulsework Knight x2 (Three separate encounters)
 * Pulsework Knight x1
 * Pulsework Knight x1, Circuitron x1
 * Pulsework Knight x1, Circuitron x2 (Two separate encounters)
 * Circuitron x3

Lower Traverse

 * Noctilucale x5
 * Phosphoric Ooze x3
 * Phosphoric Ooze x2, Noctilucale x3
 * Noctilucale x6
 * Phosphoric Ooze x3, Noctilucale x4
 * Noctilucale x10
 * Phosphoric Ooze x4

High Conflux

 * Skata'ne x1, Stikini x1 (Two separate encounters)
 * Skata'ne x1
 * Stikini x1, Skata'ne x1, Circuitron x1

Hibernatorium

 * Berserker x1 (Two separate encounters)

Inner Conduit

 * Imp x3
 * Phosphoric Ooze x2, Imp x3
 * Phosphoric Ooze x4
 * Phosphoric Ooze x2, Imp x4
 * Greater Behemoth x1 (Two separate encounters)
 * Phosphoric Ooze x3, Imp x1
 * Noctilucale x10
 * Greater Behemoth x1, Noctilucale x4
 * Greater Behemoth x1, Imp x3
 * Imp x4

Mezzanine

 * Cid Raines (Boss)
 * Circuitron x3

Central Conflux

 * Pulsework Knight x1 verses Phosphoric Ooze x2 (Three-way battle)
 * Pulsework Knight x2 verses Phosphoric Ooze x3 (Three-way battle)

The Synthrona

 * Circuitron x4
 * Pulsework Knight x4
 * Pulsework Knight x2, Stikini x1

Basement Conflux

 * Pulsework Knight x2 verses Imp x4 (Three-way battle)
 * Greater Behemoth x1

Hypogeum

 * Circuitron x4
 * Skata'ne x2, Stikini x1 verses Greater Behemoth x1 (Three-way battle)
 * Berserker x1
 * Skata'ne x2, Stikini x1

Transept

 * Berserker x1

Substratal Conflux

 * Skata'ne x2
 * Skata'ne x2, Stikini x1
 * Skata'ne x1, Stikini x2

Vaults

 * Berserker x1 (Two separate encounters)

The Apse

 * Bahamut (Eidolon battle)

Music
"Will to Fight" is the background theme of the Fifth Ark.