21st November 1916: Britannic, younger sister to the ill-fated Titanic, is steaming through calm seas in the fabled Greek islands. Suddenly an explosion rips through the world's largest ocean liner.

A mine or a torpedo is immediately suspected. This is the First World War and German submarines prowl these waters. However, a bigger mystery remains. Why did this giant ship sink so fast? In spite of extensive safety improvements, she vanishes beneath the waves in just 55 minutes, almost three times faster than her sister Titanic.

In this programme, a History Channel expedition will try to unravel the mystery of Britannic's final hour.

Wreck divers John Chatterton and Richie Kohler travel to the Greek Island of Kea. They have assembled a team of elite divers because, at 400 feet below sea level, Britannic is considered the Everest of wreck diving.

Answers to Britannic's fate will not come easily. They lie deep within the dark, rusting interior of the 900-foot wreck, in a place where no one has been before and where one wrong move could turn Britannic into an iron tomb.