A large number of dead harbor porpoises have been washing up on beaches in San Mateo County and elsewhere in the Bay Area this summer, and marine mammal experts are at a loss to explain what could be killing them.

A total of 24 harbor porpoises  12 males, 9 females, and three still in utero  have been discovered by beachcombers since early June, according to the Marine Mammal Center in Sausalito. More carcasses are likely to show up in August.

"It's the tip of the iceberg. These are open ocean animals. For every one we find dead there are probably many others that are out there," said Mary Jane Schramm, spokeswoman for the Gulf of the Farallones National Marine Sanctuary.