The New World Mall, in Bangkok, was closed in 1997 after it was found to have breached building regulations, before a fire in 1999 left it without a roof. Rainwater slowly filled the building with water, causing a major mosquito problem. In an effort to rid themselves of the pests, the locals introduced freshwater talapia fish to eat the insects. The fish thrived, quickly multiplied, and now fill the lower floors of the mall, swimming around rusty escalators and across the 5,000sqft floor. Shops around the mall sell fish food to tourists who visit, but throwing anything else into the subterranean lake is banned in order to protect the aquatic inhabitants.
Info from the DailyMail