Pat Boltz was at the lake with his daughter and wife Monday when he stepped on something sharp while wading in chest-deep water.
He found he had stepped on a long, metal spike.
"Three, four inches out of the water and they were sunk maybe a foot and a half," he said.
He rummaged around and found ten spikes stuck in the sand in the south end of the lake.
"It was booby trapped," he said. "It was like a mine field."
He then alerted the parks department.
"We were horrified," said Dewey Potter with the Seattle Parks Department. "It's probably if not the most, one of the most malicious and harmful things anyone's ever done."
Park officials called Seattle Police, and their Harbor Patrol units found another 40 or so more spikes -- although none so far in any official swimming areas.
Boltz was lucky to get only a small cut on his toe.
"That could have gone right through the ball of my foot," he said. "The one that cut me was sharpened like an arrow point. You know, it was like a razor blade."
No one else has reported being injured by the spikes.
"We're at a loss, why someone would do this," Potter said, "when the only objective could be to hurt people."
The city has put up warning signs near the swimming areas alerting people to the danger. Police are still investigating.