- [Narrator] Come spring and a warming ocean, these giants move in from deep water.

Up to 40 feet long and four tons in weight, basking sharks are the second-biggest fish on the planet.

(water whooshing) These sharks are hungry and must work hard for their dinner, for they survive on some of the smallest life in the sea, plankton.

Filtering 2,000 tons of water an hour through their giant gills.

(water whooshing) Though occasionally seen with a companion or two, the sharks usually turn up alone.

(water whooshing) Until recently, when this western edge of Ireland has seen something truly extraordinary.

The giants are gathering.

No one knows how many there are, or how these sharks find each other.

Or quite why they've come together at all.

Their mouths are closed, so food is not the answer.

Scientists think it's some kind of breeding display, a slow motion dance of desire.

Each shark sizing up suitable partners, before slipping away to mate in deep water.

We may never know for sure, for these events are incredibly rare to witness.

A brief window into prehistoric times, when the oceans teamed with life.