The hole was big enough for the tot to slip through as water eased the way. It took emergency workers 15 minutes to finally find the child after they broke through sewage pipes.
The youngster was eventually recovered and rushed to hospital in a critical condition.
The drama in the Zhaoyang District of Beijing, China, unfolded after the mum asked her husband to wait for her outside.
A similar thing happened to a 19-year-old mum in 2006 in the country's Jiang Su province. The baby made a full recovery.