The English Channel is the world's busiest waterway: 400 ships pass through the 21-mile-wide Dover Strait every day. It defines our border with Europe, but has brought us closer to it: the free flow of goods and people has transformed our economy and our lives. But as our relationship with the continent changes, these are uncertain times for the people who live and work these waters. This ambitious access-all-areas series goes inside the Channel Tunnel, on board ferries and freighters, through our highly-fortified border, and into the hidden nerve-centres that keep the whole system moving. And the maintenance workers, engineers, sea pilots and ship's captains - who move 17 million of us across the sea every year - reveal how the Channel works.