BBC Earth

Corals build themselves homes of limestone in the warm, clear, shallow seas of the tropics.

The big blue is the world's greatest wilderness, far from shore and many kilometres deep. It's a vast marine desert where there is little to eat and nowhere to hide.

It's our green seas, not the blue, that bring life to our oceans. Here sunlight powers the growth of enchanted forests of kelp, mangroves and prairies of sea grass.

On the coast, two worlds collide. Coasts are the most dynamic and challenging habitats in the ocean - that brings great rewards but also great danger.

Uncover the impact that our modern lives are having on our best-loved characters from across the series.

On the famous Ho Chi Minh Trail, Guy visits a Huey helicopter graveyard, makes his own version of napalm and comes face to face with a live cluster bomb left over from the war.