Royal Waterways

"Aren't these waterways thrilling? A labyrinth of pipes and tunnels... I couldn't have asked for a better place to employ my talents. It's all so orderly, so considered, nothing like the crude irregularity of those caverns..."

- Cornifer

The Royal Waterways are a series of sewage pipes and support structures underneath the City of Tears.

Description
Many areas in the Waterways are flooded by the water constantly dripping from above, originating from the rain in the City of Tears (and ultimately from the Blue Lake). Several rooms in the waterways are littered with piles of trash.

The area is divided into two sections by a broken lift shaft. The east part shares many traits with the west but also features acid pools and plant overgrowth.

On the western side, the NPCs Tuk and Fluke Hermit can be found. This side also contains the Junk Pit, where the  Godseeker is located. Furthermore, the bosses Dung Defender, White Defender, and Flukemarm can be found on this side too.

On the eastern side, Isma's Grove is located, where Isma's Tear can be obtained. In the centre of this side is a shortcut to the City of Tears.

Lore
The waterways' pipes were built to carry away the capital's waste. However, the incessant rain in the city above has washed the pipes clean. All trash carried by the water eventually ends up in the Junk Pit.

With the kingdom's ruin, Flukes established a colony in the depths of the waterways and populated the area along with other creatures.

How to access
The main way to access the Royal Waterways is through a hatch located in the room next to Fountain Square. The hatch can be opened using a Simple Key. It can also be first accessed by taking the Tram to Kingdom's Edge and then using Crystal Heart in a passage northwest of the Hive to end up near Isma's Grove.

Sub-area: Isma's Grove
A hidden garden accessible by using Crystal Heart to dash through a tunnel of spikes at the east end of the Waterways after defeating Dung Defender, or through a tunnel connected to the Kingdom's Edge. The Grove houses the presumably deceased Isma, on whose corpse the Knight finds Isma's Tear, which allows the Knight to swim through acid.

Isma's Grove seems to be the source of the acid that pools in the lower areas of Hallownest. The acid's origin is unknown, though there are hints that it might be actively hostile to life.

Sub-area: Junk Pit
A massive cavern filled with Geo chests, paintings, a statue identical to the one where the Monarch Wings are acquired, and assorted piles of other trash washed up by the currents. This pile of rubbish is said to be Flukemarm's treasure by Fluke Hermit who is also found within this area. Though among all the worthless junk lies a golden sarcophagus which, when opened by a Simple Key, reveals The Godseeker where she has been hibernating. Accessible by breaking a roof on the northwest corner of the square-like area next to Flukemarm's boss room. A wall at the top of the Junk Pit can be broken to reveal access to the City of Tears, below the Nailsmith's forge. If the Nailsmith is killed, his corpse appears afloat the Junkpit waters, near the shore.

Trivia

 * There is an unused key in the game's files labelled "Waterways_Key" which was presumably used to open the Royal Waterways instead of the currently used Simple Key.