When someone with a pneumothorax lies supine -- as in the 23-year old man described in this New England Journal mini-case from Saweera Sabbar and Eric James Nilles of Rashid Trauma Center in Dubai, UAE -- air rises laterally and caudally, and creates displacement downward and medially of the hemidiaphragm, displaying as the "deep sulcus sign" on radiographs. Here's a nice picture of how it looks.

Sabbar S, Nilles EJ. N Engl J Med February 2012.