Detective Melanie Robinson of the Kent police said the incident unfolded at the Waterford Apartments, at the corner of 236th Street and Lakeside Boulevard East, at around 4:20 p.m. Sunday.
Robinson said the family was celebrating the little girl's second birthday at the apartment complex's recreation center with a large number of family members present when they suddenly noticed that the little girl was no longer there.
They quickly searched the immediate area and discovered her face down in a large pond behind the building.
Police and medics responded to the scene and began CPR on the little girl, who was in cardiac arrest. After about 45 minutes, medics believed they may have gotten a very faint pulse so they rushed the girl to Harborview Medical Center in Seattle.
From there, the girl was taken to Seattle Children's Hospital, where her condition remains uncertain. She is still unconscious, and it is unknown what the long-term impacts will be.
Robinson said police determined no crime occurred - just a "very tragic incident."
The pond where the girl was found is one of several in the area of the 23600 block of Lakeside Boulevard East, where several apartment complexes are located. It is unknown how long the little girl had been underwater.