Around 20% of the world’s oil supply normally passes through the Strait of Hormuz - the narrow waterway between Iran and the horn-like tip of the Arabian peninsula.
Right now it is effectively closed, with Iran deciding which ships can get through, and the impact is being felt worldwide.
The BBC's Diplomatic Correspondent Paul Adams explains why navigating this key shipping channel could be so dangerous.
- Read more on this story: Why the Strait of Hormuz matters so much in the Iran war
