I was recently in [Baie d'Oro, Ile des Pins, New Caledonia](https://www.google.com.au/maps/place/Oro+Bay/@-22.5761769,167.509862,15z/data=!4m5!3m4!1s0x6c29be5786aa4a45:0x950095adc1b199af!8m2!3d-22.566943!4d167.5232461), and the lagoon had dozens of these mysterious green balls rolling around the sea bed:

[![enter image description here](https://i.stack.imgur.com/oZn2c.jpg)](https://i.stack.imgur.com/oZn2c.jpg)

They varied in size from large grape to small grape, with "skin" less than a millimeter thick and seawater inside, which would squirt out if you squeezed them hard enough or (as the kids soon discovered!) burst if you threw them like snowballs.

**What are they?** My best guess would be some kind of seaweed sac, but I never saw them attached to anything and they weren't air bladders, since the balls did not float. They're also not [marimo](https://en.wikipedia.org/wiki/Marimo), which are solid seaweed.