"There are years when blooms or populations of a jellyfish are high and there are also years when they are few or even almost absent," said Sheldon Rey Boco, who filmed the event.
Upside-down jellyfish are found in warm coastal waters—such as mangroves, bays and lagoons—around the world.
"If we can find a way to direct these jellyfish and also equip them with sensors...we could create a truly global ocean network," said lead Caltech researcher John Dabiri.
The venom of some box jellyfish species is considered to be among the most powerful in the world.
While jellyfish are fairly common in the area, such large congregations are quite unusual at this time of year.
Some species of jellyfish are able to grow back tissue that has been lost.
Jellyfish larvae can get trapped in bathing suits and sting the skin.
The creatures pack a powerful sting that can be excruciatingly painful to humans, and in very rare cases, may cause death.
Study says the finding of an "intermittent anus" may shed light on the evolution of our own.
Sea lice are not actually lice; they are the larvae of jellyfish and sea anemones.
Summer winds left the Catalan shoreline covered in thousands of blue jellyfish-like animals.