Just when you thought it was safe to go back into the water.

On Friday, October 16, the U.S. Coast Guard spotted a cluster of 20 or so great white sharks swimming about 100 yards off Ocean Beach and Pacifica. Most of the sharks were between 10 and 15 feet long, but a few "really big" individuals were up to 18 feet long. "This is the first I'd heard of near-shore aggregating in such an urban area," Mary Jane Schramm, spokeswoman for the Gulf of the Farallones National Marine Sanctuary, told SFGate.

"An unusual number of juvenile white sharks under 10 feet long have been observed this year, likely associated with the unseasonably high water temperatures along the coast," David McGuire, Director of the San Francisco shark conservation group Shark Stewards said din a press release. Earlier this month tourists on Alcatraz were treated to the rare sighting of a shark feeding on a seal inside San Francisco Bay.

"But an 18 foot shark is very large, mature shark and can cause considerable harm to seals or humans," he added. The larger, mature sharks are typically in the waters off the Farallon Islands, Año Nuevo or off Drakes Bay, feeding on their preferred prey of elephant seals and sea lions.

Despite their natural presence in our waters, shark attacks on humans are still very rare. However, because of the unusual concentration and proximity to the coast, McGuire cautions surfers, swimmers or kayakers for the time being.

"I might swim and surf somewhere else for a few days."

No word on whether or not this shark gang's rivals were also spotted nearby.