This question has spoilers for Riven, since it requires explaining one of the puzzles in the game in some detail.

In the 1997 puzzle adventure game, Riven: The sequel to Myst, one prominent puzzle involves a boiler the size of a small building:

The boiler house on Book Assembly Island in Riven. Copyright Cyan Worlds.

To solve the puzzle, the player needs to figure out how to turn off the heat, drain the water, and raise the metal floor inside, allowing them to enter the building and crawl into a large metal pipe. You can see this pipe in the image above - it starts underneath the round boiler house, goes horizontally at ground level towards the left, and then up over the crater ridge.

In the image you can also see two large striped hoses going from the lake towards the boiler. The one to the right appears to connect to the large metal pipe. The other one makes a gentle curve up along the left side of the boiler, and into what appears to be a large pump. Here's a close up of the area next to that pump:

Boiler house controls

These are the main controls used for solving the puzzle. The large rubber hose on the left can be connected or disconnected by turning the metal wheel. When it is connected, the boiler is filled up. When it is disconnected (as in the image), the boiler is drained. For reference, you can see the full puzzle being solved in this video: https://youtu.be/uOLr5rUDIOs?t=316. The water is drained around 6:50.

What's so puzzling about this puzzle is: why would disconnecting the rubber hose drain water from the boiler?

Note that the water doesn't exit through the rubber hose, which is maybe what you would expect, since it is disconnected. In fact, it's a little unclear exactly where the water goes, but there are reasons to believe it exits through the large metal pipe, since you can see a puddle of water where it ends:

Puddle of water beneath the end of the metal pipe

If that's the case, the water has moved up over the ridge of the crater, exiting maybe 30 ft higher than it started. That must require quite a lot of power to achieve.

Another thing to note is that it's not possible to drain (or fill) unless power has been directed to the boiler from a control in the middle of the lake. The rubber hose can be connected or disconnected when power is off, but it has no effect on the water. That perhaps suggests that the pump is used to drain the water. But the pump is attached to the rubber hose, which is what you disconnect to drain the water.

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    $\begingroup$ Welcome to Puzzling! I'm not sure this is really on-topic here -- the question seems to be more about the game mechanics and the video game designers' intentions rather than the puzzle itself. (It's a question about the game's universe and physics more than the abstract puzzle, unless I'm misunderstanding.) You might have more luck over on Arqade? Their "myst" tag seems to have a few questions like this. $\endgroup$ – Deusovi Feb 27 at 8:44
  • $\begingroup$ @Deusovi I was unsure if it would be on topic here, but I felt more confident after reading the help page: "If you have a question about [...] A well-known (perhaps unsolved) puzzle [...] then this site is the right place for you!" That seems to describe my question perfectly: a question about a well known puzzle. $\endgroup$ – last-child Feb 27 at 8:50
  • $\begingroup$ @Deusovi As you are a moderator, perhaps you could help me with this procedure, if you think it is appropriate? meta.stackexchange.com/a/85041 $\endgroup$ – last-child Feb 27 at 8:59
  • $\begingroup$ Sure, there you go! Hope you get an answer there! $\endgroup$ – Deusovi Feb 27 at 9:40
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    $\begingroup$ Based on everything you've described, it would seem that the valve that connects and disconnects the hose also activates and deactivates pumping of water out of the boiler. Either the hose is connected and the pumping out is deactivated, or the hose is disconnected and the pumping out is activated. Mechanically, this doesn't make much sense. Possibly what the designers were thinking is that the boiler is constantly pumping out water (once powered) and that hooking up the hose simply pumps water into the boiler faster than it can be pumped out. $\endgroup$ – COTO Feb 27 at 14:55