Ship operators rely on a sludgelike substance known as bunker fuel to keep vessels running. The Iran war 's closure of the Strait of Hormuz has choked off the supply of this fuel that powers the global maritime industry and its largest refueling hub in Asia.
Bunker fuel is a literal bottom of the barrel product — heavier and dirtier than the more expensive kinds of refined crude oil used by other vehicles like cars and airplanes — it sinks to the bottom of storage containers.
