Loveland officials first thought it was a joke, even after several people had reported seeing a large reptile in the Jayhawker Ponds.
Now the ponds and the 30-acre natural recreation area around it are closed until the animal is caught.
Larry Callihan, Love's parks manager, said a Larimer Humane Society confirmed the animal, estimated to be a a five-foot to seven-foot-long reptile, is in the pond.
State Wildlife officer Aimee Ryel said they will use an electric shocking boat to try to get the animal to surface.
Police Sergeant Rae Bontz said as long as people stay away from the area they will be safe.