The Greenpeace action where six women climbed to the top of the Shard in London.

I was in London that day, quite near the Shard. And all I could see was helicopters circling The Shard.

It was only when I checked Twitter that I realised what was happening.

11 July 2013, six climbers began a 15 hour climb of Europe's tallest building, the Shard, to send a message to the headquarters of oil giant Shell.

This behind-the-scenes film captures the months-long build-up to the climb and the first hair-raising moments when things didn't go to plan.

The climb was live-streamed from the activists' helmet cameras in a ground-breaking digital campaign. A live audio commentary overlaid the footage for the whole day -- with Greenpeace presenters interviewing experts and taking comments from as far away as New Zealand. #IceClimb trended globally on Twitter and at one point six of the top ten trending topics in the UK were about the protest.

Watching the film back today gave me goosebumps.

The climbers: Ali Garrigan, 27 (UK), Liesbeth Deddens, 31 (Netherlands), Sabine Huyghe, 33 (Belgium), Sandra Lamborn, 29 (Sweden), Victoria Henry, 32 (Canada), Wiola Smul, 23 (Poland).

Heroes.