A military veteran who said his skull was crushed by a 16-pound (7-kg) pine cone as he rested in the shade of a conifer grove at a San Francisco park has sued the U.S. government, saying employee negligence led to his injuries.

Sean Mace was reading and napping beneath the bunya pines, also known as false monkey puzzle trees, during a visit to San Francisco Maritime National Historic Park in October 2014, according to the lawsuit, filed in federal court.

The cones dropped by the Australian trees can grow in excess of 14 inches (35 cm) in diameter and weigh more than 40 pounds (18 kg). Park staffers who planted the trees decades ago knew there was a high risk of injury to anyone below them, the complaint said.