I have an air breathing species that, for reasons of Plot, spend most of their time underwater. They’re pretty much comparable to humans in everything except how long they can hold their breath. Twenty minutes is the average. Only children and the disabled can hold it for less than five minutes.
This particular race live in the submerged, upturned ruins of a past great civilisation. Various air pockets have formed in the ruins submerged in the shallow waters on the coasts of their world which they can use as shelter from the various nasties on the main land masses and also the things that lurk in the deep. These ruins are airtight and pretty much indestructible, so if left undisturbed the air pockets would last a long time.
This race is smart, but pre-industrial, and the air in their homes is obviously finite. Any attempt to equalise the pressure with the surface using hoses etc will flood their homes, so the question is this:
How can this race cycle bad air out of their homes and good air in, using pre-industrial tech
I know some species of spider will capture bubbles of air from the surface and drag them down to their nests to keep a bubble of air for themselves. Could this be a viable tactic for much larger creatures? If the trip to the surface is short enough could the large lung capacity and low oxygen use of this species mean exhaling right after coming from the surface is good enough to refresh the air in their homes for a while?