After falling for Don Corneo's trap, Cloud and company are thrust into the sewers, whereupon they are assaulted by Corneo's pet, Aps. After defeating Aps, Tifa loses hope in Sector 7's plate still being intact, but is reassured by Aeris that it is not too late in saving everyone. They find a way out and emerge in the Train Graveyard.
The Sewer System is located beneath Sector 6 and the Sector 7 Slums, connecting Wall Market to the Train Graveyard of the Sector 7 Slums. It is divided into two portions: the Sector 6 Sewers, and the Sector 7 Sewers. The Sewer System comprises aqueducts of raw sewage running beneath the Midgar undercity, and has mostly brick walls. Residing in the raw sewage are monsters, and Don Corneo's pet, Abzu. Across the aqueducts are sluice gates to control the flow of water.
In Final Fantasy VII Remake, between the Sector 6 and Sector 7 plates is the Trunk Line, also known as the Sector 7-6 Main Corridor. In contrast to the rest of the sewers, this vast room is almost entirely metal, with walkways at the top forming a path between the sectors across the aqueduct at the bottom. There are also control rooms present to oversee the flow of sewage over the canals, though these rooms appear to have undergone little to no maintenance and seem largely damaged. Finally, the sewers serve as a hideout for criminals, including Corneo, who has stashes and shelters within the sewers.
Final Fantasy VII
The Sewer area is visited in "Prevent the Fall of the Plate" by a party of Cloud, Aeris, and Tifa after Don Corneo ejects them through the trapdoor in his Wall Market bedroom. Cloud can help up either Aeris or Tifa, affecting date mechanics, and then a battle with Aps ensues. After making their way through the sewers, the party arrives in the Train Graveyard.
The field map is an unrevisitable location, and is made of only two screens, one after they fall in, and one toward the exit. Aside from Aps, a few enemies are fought, in the form of the Ceasars and Sahagins. However, the area is very brief.
Final Fantasy VII Remake
The Sewer System is first visited in Chapter 10, "Rough Waters," by a party of Cloud Strife, Aerith Gainsborough, and Tifa Lockhart. As in Final Fantasy VII, the three fall in from Wall Market, Cloud can help either Tifa or Aerith get up to affect affection mechanics, a battle with Corneo's pet Abzu ensues, and the three navigate through to reach the Train Graveyard on their way to the Sector 7 Slums.
A new segment of the game appears in Chapter 14, "In Search of Hope." In this, Cloud, Tifa, and Barret Wallace follow Leslie Kyle to help resolve a personal matter with Don Corneo, in exchange for Leslie's help reaching the Shinra Building.
In addition to the new quest in Chapter 14, the Sewer System is greatly expanded compared to the original Final Fantasy VII, as the location is a more complex series of tunnels and aqueducts. The Trunk Line is also added, a major aqueduct that marks the point dividing Sector 6 and Sector 7. Sahagin are still the main enemies, though others are fought, including blugu. Furthermore, in addition to Abzu, Abzu shoats are also fought.
The theme "Lurking in the Darkness" (闇に潜む, Yami ni Hisomu?) plays in the sewers.