As she walked along Long Beach in Sandy Bay, Tasmania, veterinarian Grace Wherrett stumbled on an unlikely scene. She spotted a lone seal, splashing in bioluminescent waters, ducking in and out of the water as it caught fish. The keen Instagrammer managed to capture the scene on film. Bioluminescence is usually caused when algae blooms in shallow waters, causing the sea to glow at night. In order for the algae to bloom, the ocean must be relatively calm and warm. Unless the water is disturbed by the movement of waves - or in this case by a playful seal - the glow is not seen. It's only once the water moves that the otherwise invisible lightshow is revealed. But its beauty hides its danger. According to a report by National Geographic, bioluminescent water can prove harmful to sea life and humans. The algae that causes the glow can poison anything that comes into close contact with it. Though, a small amount of contact can be fine. For this reason, it's not advised to swim in bioluminescent waters.