Hives were covered with red biomass that would congregate towards the bottom of the building. The effects of a Hive's presence could be felt blocks away, as it served as a makeshift Infected "Head-Quarters", and Infected gathered around the structure to defend it from invaders. They were also used as incubators for the creation of infected walkers and Hunters.
During the mission in which Alex enters a Hive, papers and other office materials may be found strewn on the ground. This suggests that the Hives used to be ordinary buildings before they were infected.
Despite the fact a tank can destroy a Hive in less than a minute, the military is rarely able to take a Hive down without Mercer's assistance.