# How would a w shaped tunnel system drain water?

My DnD group recently had a scene where we were escaping a tunnel system under a lake. My character used a bag of holding to slow drain the lake until we were at the bottom near the entrance of a tunnel. Picture a sink with the drain at the bottom. The tunnel system basically snakes down and up like an exagerrated W, with a pocket of air at the top and us starting on the right top side.

My idea was that I should be able to elevator us down and then up each bend by swallowing / releasing water from the bag. The DM figured that the water on the other side of the bend would stay up and collapse onto us as we reached the bottom of the first bend. How would the water behave in this scenario? Does it depend on whether the space between bends is air-tight or not?

• What’s a “bag of holding”? Was this a google translate result? – Solar Mike Mar 7 at 20:32
• Drain using a syphon, at least until the air breaks it. – Solar Mike Mar 7 at 20:33
• @SolarMike a magical bag that has infinite interior space and weighs the same no matter how much it holds. – Alex Eggers Mar 7 at 21:41
• Great you must have a Tardis then.; How is it? – Solar Mike Mar 7 at 21:52
• In order for your idea to work, the bag must have the same volume as half of the water in the tunnel. The cross section of the tunnel is finite, how can it accommodate something (the bag) much larger than its size? – r13 Mar 8 at 0:52