Apparent shopping carts — dozens of them — have been spotted submerged in the Colma Creek marsh in South San Francisco, visible only when the tide drops. Save the Bay calls it the densest concentration ever seen in one place on the Bay shoreline.

At low tide, the Colma Creek marsh in South San Francisco shows something that doesn't belong: what appears to be dozens of shopping carts, half-buried in the mudflat where the creek meets the San Francisco Bay.

The carts span the mouth of the marsh alongside the SF Bay Trail, the paved path that curves around the South San Francisco shoreline. At high tide, they vanish. At low tide, trail regulars say they initially mistook the metal frames for rocks — they blend into the flat at a glance, and only resolve into carts when you stop and stare.

"I've never seen grocery carts out in the Bay like this," one trail user told ABC7 reporter Stephanie Sierra this week.

Josh Quigley, Save the Bay's Senior Policy Manager, confirmed the scale. "This is certainly the greater concentration that I've ever seen in one place," he said — notable given that Save the Bay, founded in 1961, has been tracking Bay shoreline pollution for decades. Quigley characterized the discovery with measured frustration: "The bay is treated not as the jewel and resource that it should be, but as a dumping ground."

How the carts got there remains open. A Costco Business Center sits roughly a mile from the marsh, a proximity that trail walkers noted aloud — the carts do appear oversized, consistent with warehouse models — though confident identification at low-tide distance isn't possible. The South San Francisco store's general manager told ABC7 that employees walk and clean the trail "multiple times a week, in addition to checking for any carts that might end up in the bay."

ABC7 contacted the San Francisco Bay Regional Water Quality Control Board to push for cleanup. Save the Bay says overall Bay shoreline pollution has been declining — which is what makes a concentration this dense worth flagging.

No removal is scheduled yet. Twice a day, the tide rises, and the carts disappear again.