Answer: Body Heat

The Mall of America, located in Bloomington, Minnesota, is the largest mall in the United States with over 4.8 million gross square feet of space contained within it, sprawling across four floors. With that much square footage, you’d assume that it must have an absolutely massive heating system to keep it warm in the chilly Minnesota winters, right? While some individual stores within the mall have small heating systems to adjust their particular temperatures (such as making a spa warmer than other spaces), the mall itself has no central heating system.

Mall architects took into account the heat gained from the numerous skylights, the waste heat from the lighting, and—most importantly—the enormous amount of heat generated by the tens of thousands of shoppers in the facility at any given time. From the Mall of America “About” page:

MOA does not use a central heating system; instead, 70 degrees is maintained year-round with passive solar energy from 1.2 miles of skylights and heat generated from lighting, store fixtures and body heat.

Shoppers generate so much extra heat, in fact, that during peak shopping in the winter season (when temperatures can dip into the single digits in Bloomington), the mall actually runs the air conditioning system to keep the facility comfortably cool.