Airfoils work by being shaped specifically to generate a certain pressure difference on the top and bottom of the structure to produce lift. If you change the density of the air you are slicing through, the relative pressure (top and bottom of the wing) changes and you lose your lift.
Also the altimeter works on air density, so it will show you as climbing when you may actually be in a steep descent.
I see. Methane in water affects the air density at the altitudes planes fly at. Hmmm....
BTW, I am well aware of how an airfoil works.
(Note to all: I do not buy into "The Bermuda Triangle")