What’s green, spongy, and washed up on the beach? No, I’m literally asking: what’s green, spongy, and washed up on the beach? Because nobody seems to know.

One member of the Dee Why Surf Life Saving Club remarked (with tongue almost certainly in cheek): “They look like alien eggs or something.” The name stuck, and now almost every headline about these little Tribble-like creatures fits the phrase “alien eggs” in somewhere. But they’re clearly not eggs—they’re fibrous and porous and appear to be made out of seaweed—and they’re probably not alien. So what are they?

History suggests one obvious candidate: the marimo or Japanese moss algae, better known to almost nobody as aegagropila linnaei. The marimo are such a big deal in Japan that they actually appeared on a postage stamp, but you don’t generally see them wash up by the hundreds on the shore in Australia. Also, as one beachcomber remarked: the Sydney balls seem to be made of seaweed, not moss (though Japanese moss balls are technically made of algae, not moss, so it’s possible this is just what they look like when they dry out).

This 1956 Japanese postage stamp depicts the distinctive marimo (Japanese moss balls), a species of aegagropilae (ball algae).

Whether the balls are semi-desiccated marimo or a less well-known species of ball algae, local marine biologist Alistair Poore is pretty sure they’re alive:

“I’ve seen similar things — sometimes dead sea grass can roll around and form balls like underwater tumbleweeds but that’s made of dead material and these look to be living. I think you’d find that they’re very likely to be the green algae.”

Poore and his colleagues are no doubt working on a more specific explanation as we speak. Meanwhile, I think some wild speculation is in order. You can share your best (or worst) guess on what they might be in the comments below.