A fuel tank.
In 1941 in Kangerlussuaq, the Americans, in cooperation with the Danish government, set up a military base to support activities during World War II. The airport here played an important role in the North Atlantic ferry route between America and Europe. During the Korean War and the Cold War, it served as a resupply station for the DYE 1, 2, 3 and 4 Distant Early Warning Line bases. At its peak, 1,400 workers were stationed at the base.
After the collapse of the Soviet Union, in 1992, the United States sold the base and the airport to the Greenlandic government for a symbolic amount of US $1. Their condition was to be allowed to conduct military exercises in this unique part of the world. Now, the former military airport is used for commercial passenger flights and is the main transit point in Greenland.
Huge fuel tanks, such as the one shown in this photo, are a relic of the airport's former character. Or at least that's what I think, as MOGAS refers to MOtor GASoline, as opposed to AVGAS, AViation GASoline, plus the tanks were empty, which suggests that they are no longer in use.