(CNN) Police in Australia are urging the public to stay alert after a runner found homemade spikes methodically hidden on a popular running trail Monday.

The man was on Alexanders Trail in Lake Macquarie, a city in New South Wales, when he noticed a pile of leaves in the middle of the track.

He found wine corks with nails protruding from them, police said in a statement posted to Facebook. They resembled caltrops, illegal weapons that have at least two sharp points sticking out of a base.

The homemade caltrops were found on Alexanders Trail in Lake Macquarie.

He continued along the trail and found another pile of leaves with more spikes hidden. Further along, he found another.

Police are investigating.