It is a heavy gas that settles close to the earth and, like water, seeks its lowest level.

It was something entirely different, a heavy gas perhaps.

The light moved like smoke, or heavy gas not in straight lines, but floating ribbons..

Radon is a heavy gas, and the later products are solids.

It could even be some heavy gas - no, I suppose that's impossible.

Radon is a heavy gas and thus will tend to accumulate at the floor level.

It is a heavy gas and will tend to seek low areas.

The heavy gas of the cloud was beginning to oppress him now.

It had almost no atmosphere, just a thin layer of the heavier gases.

Fortunately it's a heavy gas and stays close to the ground, which was why you remained safe on your hill.