Sea Trek - Episode Guide

As inquisitive sealions mob the strangely garbed divers, their boisterous play threatens the delicate cameras and cables.

These tropical islands are full of surprises - penguins fly past colourful coral gardens in pursuit of clouds of tiny fish. At high tide, the explorers follow marine iguanas through pounding surf to watch them feed on submerged seaweed. And, in deeper water, they hold their breath as the hammerhead sharks arrive.

In their second film, divers Martha Holmes and Mike de Gruy sample the Caribbean, where they enjoy close and loving encounters with the sea creatures in the waters around the Cayman Islands and the Bahamas. Paradise, perhaps, but there's a sting in the tail when they are mobbed by a squadron of giant sting-rays, who administer love-bites on their legs. But friendly schools of dolphins are happy to play, though only when the divers substitute snorkels for their bubble helmets, whose weight acted as a brake on the underwater frolics. Until the humans can match them dive for dive, the lively dolphins find them rather boring.

In this third film bubblehelmeted divers Martha Holmes and Mike deGruy dive beyond the pounding Californian surf to a submarine forest of breathtaking beauty - and predatory blue sharks. The underwater trees are a type of seaweed, giant kelp. Through sunlit glades the divers find harbour seals, bat rays, clouds of colourful fish and sea otters.

Just beyond the kelp, the angel shark proves to be no angel, while further out in deep water, a dive with blue sharks calls for heavy protection.

Divers Martha Holmes and Mike de Gruy head for Australia, for an event described as "the greatest sex show on earth" - the mass spawning of corals. While waiting for the curtain to rise, they are surrounded by circling grey reef sharks. In a less sinister encounter they face a posse of giant groupers. Up on the beach, the divers watch entranced as turtles lumber ashore to lay eggs. Then, in a fitting climax on the chosen moonlit night, the corals all along this spectacular reef release their eggs and sperm in an underwater extravaganza to rival any firework display.

A whale shark looms out of the blue, dwarfing divers Martha Holmes and Mike de Gruy. It's a moment of high drama, a breathtaking encounter with the biggest fish in the ocean. But the intrepid pair came to these Pacific islands to track down another heavyweight - the humpback whale. Surrounded by the mysterious sound of singing whales, their hopes rise. Their persistence pays off when a 35-ton mother with a newborn baby lingers quite unconcerned in front of the camera. For the divers it's the highlight of an eventful trip, and a fitting finale to the Sea Trek voyages.