Remnants of a water-purification system? A system put in place by a prescient government in the days before the Flood? Ruins of a cult's greatest work, ever reaching, ever searching through the Deep? Ask someone on the metal walkways or stone streets of Weepingfort, and you'll get a different answer each time you ask what the Drowning Tubes used to be. It doesn't matter anymore. Now, they're the best means of transportation to get to any part of the Deep, quick as you please.
Entrances to the Drowning Tubes dot Weepingfort's map. Workers' entrances scattered throughout town are often cramped and busy, and you may have to wait while deep sea divers and other grinders fight their way to the next descent, hoping to clock in on time. Tubes opening in the Cave Districts pretend to be more civilized, with marginally more comfortable descents featuring seats and straps instead of handholds, and often have the most direct routes to the most beautiful parts of the Deep, if not the safest. And, of course, there are the secret entrances, the hack-and-slashed openings, cobbled-together latches and descents, that the scavengers use. If they come back, they bring with them the curious wonders and useless oddities from the days before the Flood Age, old technologies and waterlogged memories.
Some say the fort atop the mountain for which Weepingfort is named has its own Drowning Tube, leading to the maids-o'-the-depths' deepwater cities, to the most forgotten of the forgotten pre-Flood Age civilizations, to the other side of the world where the air is fresh and the sun is hot. While the Watchers at the fort do nothing obvious to quell these rumors, they do nothing obvious to confirm them, either.
All the Tubes snake around and beneath the city, connecting with other Tubes for a quick jaunt to another Weepingfort district or section of the Deep. How far do they go?
The Tubes are thick sheets of metal at most parts, most efficient for high-speed low-depth travel. At some descents' exit points, glass walkways allow the Watchers and curious travelers to walk around the Deep and observe in a relatively safe fashion.
The Drowning Tubes were from a time before Weepingfort. The commonly accepted answer is that they were part of an extensive water purification system supplying the outlying towns and villages on the continent before those civilizations were destroyed during the Flood Age. As Weepingfort thrived, the Drowning Tubes were constantly repurposed and reused to access the Deep and its resources. It wasn't long before the current network of transportation became as everyday as rain.