Sea urchins gather in large enough numbers to form an army.

Subscribe: http://bit.ly/BBCEarthSub

WATCH MORE:
New on Earth: https://bit.ly/2M3La96
Oceanscapes: https://bit.ly/2Hmd2kZ
Wild Thailand: https://bit.ly/2kR7lmh"

Welcome to BBC EARTH! The world is an amazing place full of stories, beauty and natural wonder. Here you'll find 50 years worth of astounding, entertaining, thoughtprovoking and educational natural history content. Dramatic, rare, and exclusive, nature doesn't get more exciting than this.

This is a commercial channel from BBC Studios. Service & Feedback https://www.bbcstudios.com/contact/co...