Through the Underground Waterway, The Hideout are quests in Chapter 14, "In Search of Hope", in Final Fantasy VII Remake, taking place in the Sewer System. Cloud Strife, Tifa Lockhart, and Barret Wallace agree to help Leslie in exchange for information on how to reach the Shinra Building.
This quest and the following are required to complete the sidequest "Corneo's Secret Stash".
Walkthrough[]
When the quest begins, follow Leslie, and he will then ask the party to lead the way to the Trunk Line. The path here is similar to that from "Follow the Waterway", though more debris blocks the path forward. As the quest starts, run forward and over the debris to the upper level, then head down the steps above to the empty canal and under the sluice. A sahagin is fought. It can occasionally use Sahagin Curse to inflict 
Toad on a character, and its Jump attacks can be deadly if not guarded against. Defeat it with character abilities, or with spells from the 
Fire Materia. Use the ladder on the right to proceed to a room with a bench and vending machine. Use the bench to heal, and then proceed.
Proceed down a linear path from the vending machine then down a ladder, and head left to a room with a sahagin and blugus. The blugus are a higher priority to destroy first as they can inflict 
Sleep; deal with them before focusing on the sahagin. In the left room, wererats are fought alongside scissorclaws, all of which can be destroyed with character abilities, though scissorclaws can also be defeated quickly with spells from the Ice Materia. To the right is a room with sahagin and scissorclaws, wherein using fire magic at enemies while they are clustered together can be effective.
After defeating them, head down a path on the right, and a previously locked gate with Corneo's logo on the front is now open. Proceed down this gate and up a ladder to a previously unseen area. Follow the linear path and defeat the scissorclaws that appear, then head down the ladder on the left and obtain the ether from the chest. Continue through the tunnel and defeat the scissorclaws and wererats, then use the ladder up right to proceed down the path, leading to the trunk line.
At the trunk line, follow Leslie right to a door and approach him. After some dialogue, head down the path past the unlocked door and defeat the scissorclaws ahead. Head right and open the door then proceed down the corridor, and Leslie will note the group is close to Corneo's hideout. Through a corridor, heal HP/MP at the bench, and the disc for "The Chase" can be purchased. Head right and approach Leslie, leading to a cutscene, following which "Recovering the Key" commences.
Hard mode tips[]
Conserving MP is ideal throughout these quests. Therefore, avoid casting spells, and use 
Prayer Materia. 
Elemental Materia can be paired interchangeably with fire and ice materia. 
Parry Materia is ideal to avoid the sahagin's Jump attacks for Tifa, while Cloud and Barret equip Steadfast Block Materia.