Answer: Koi and Tilapia

When you think about things abandoned malls are filled with, you might think of graffiti, crime, or just piles of trash. In Bangkok, however, there is an enormous mall filled with something far more interesting: thousands upon thousands of koi and tilapia fish.

Built in the early 1980s, the New World Mall in Bangkok had a short run as a mall before officials discovered that owners of the 11-floor mall had only secured zoning permissions and approval for the first four floors. Demolition of the the upper floors and a fire left the building exposed (as the owners abandoned the building and made no effort to rebuild it). Exposure to the elements led to significant flooding in the building and the ground floor essentially became an enormous pond. Where there is standing water there are, inevitably, pests that arise from it and the mall-turned-pond was no exception. After the pond became an expansive breeding ground for mosquitoes, locals began seeding the flooded mall with koi and tilapia to deal with the problem (both fish species are voracious and will happily eat thousands of waterborne mosquito larva a day).

Today the mall is still flooded, but the mosquito population is well under control thanks to the huge schools of fish that devour any larva laid in the water.