They did experiments and discovered that right around 1% methane will kill a
radial engine. They also had to speculate, though the speculation was proved
to be plausible, that enormous erruptions of methane gas are more than possible
in that area, large enough to change the density of air. Since altimeters read
relative air pressure, if the air suddenly becomes thinner, two things happen--
the plane looses lift and the altimeter indicates a higher altitude. At night,
with no horizon, a pilot relying on his altimeter might think he's suddenly
climbing and push the nose over. Since he's actually losing altitude already,
the plane dives and smacks into the ocean.
In the scenario where methane is not present in sufficient quantity to cause
the pilot to dive, but is present enough to make the engine konk out, several
pilots who survived these incidents did report sudden, unexplained engine
failure. And since all 5 aircraft in question are located in an area known to
have methane eruptions, this is probably the explanation.
The bit about ships foundering has to do with a ship entering an area just as
one of these methane pockets erupts. The net effect is the water is
momentarily no longer able to support the ship-- the bow or stern of the ship
is swamped and the ship either breaks its back or capsizes.