Scenario + question: So for the purposes of this question lets so that there is a canyon much like the grand canyon but about 50 miles deeper, so the bottom is always in shadow and it has no observable bottom (but there is one). What kind of gas would it need to be filled with for floating islands to be possible?
Requirement: This is in a world that has natural magic (as in occurs naturally and by itself), however I would like to make it at least kind of scientific. The gas might be so dense that it appears on on the bottom of the canyon in a liquid state, which is OK. The rock may also have another gas trapped inside (e.g. helium or oxygen), however it needs to be solid enough for a building to exist on the top (more like huts because of the time period).