It was just a whole lot of dead jellyfish -- thousands of them -- that washed ashore near Pacheco Street. From the Chronicle:
A marine biologist at the Aquarium of the Bay at Pier 39 told the Chronicle that jellyfish have been washing ashore around the Bay for 500 million years or more.
More than 10,000 of the gooey invertebrates, each about the size of a dinner plate, drifted onto the beach Friday evening. By Sunday, they had attracted hordes of the curious, the repulsed and the fascinated.
Kids stomped them. Dogs rolled in them. Surfers tossed them at each other. Some people tiptoed fearfully around them, while others pressed in with cameras for close-ups. Read full article.
Photos and videos at links below.