A cave digged into the side of a sand dune collapsed, killing the boy.
The 10-year-old boy and his brother were tunneling under a 40-foot (12.19-meter) cypress tree at crowded Salmon Creek State Beach on Sunday, fire officials said.
"The tree did fall over, but it is purely speculation if the wind or age or tunneling brought the tree down," Bodega Bay Fire Chief Sean Grinnell said.
The older boy was partially buried but was able to dig himself out, and he ran for help, Grinnell said.
It took more than an hour for rescue personnel to pull the 10-year-old out of six to eight feet (1.83 to 2.44 meters) of sand. His identity was not immediately released.