The Seattle Times says the Parks Department placed the spikes in the lake more than 20 years ago to hold down plastic sheeting in an attempt to control water plants.
The metal stakes originally had curved tops that rusted off, leaving sharp points.
As police were investigating, some park users really began to worry after dozens of spikes were found in the popular swimming spot.
At the time, Parks spokeswoman Dewey Potter said she couldn't think of any reason for the spikes.
She said she suspected someone was trying to hurt people.
When the real reason came out, Potter said "I'm sorry it was us."
Police have closed their investigation.
