Dark ships and shadow fleets – what is crossing 'closed' Strait of Hormuz?

Iran's move to shut the Strait of Hormuz has brought shipping through the 24-mile stretch of water close to a standstill.

Only a handful of vessels have transited in recent days, many switching off their tracking systems or linked to "shadow fleets". "They can't physically close a waterway of that size, but the threat is there.