Watch NOVA Season 30 Episode 15 Online

In 1989 marine biologist Alexandre Meinesz went diving off southern France and was stunned by what he saw: a dense blanket of waving green fronds stretching around him in every direction on the seabed. At first Meinesz had no idea what it was. Then he made the alarming discovery that a tropical alga had taken root in the cold water of the Mediterranean, wiping out native sea life wherever it grew. "Deep Sea Invasion" follows Meinesz on his scientific detective hunt to discover the source of this deadly organism, his uphill battle to alert authorities to its danger, and the struggle to find a non-toxic way to control it.