2023
Zombies
There are several different types of Zombies, and scholars have tried to study the different types so they can identify what makes each type tick (ironically, most technical knowledge about Zombies is rumored to come from STOP).
The most common type of zombie just seems to lumber around, looking for food to eat. Occasionally one will find a large piece of meat and unintentionally let others know via noise. These zombies are slow and easy to deal with. It seems that destroying the front of their brains disables them, and they just sit and rot and can be forgotten.
However, an estimated .01 percent of zombies are "wakers" and will seem to be disabled for periods that sometimes extend into weeks. Then, they will get back up and continue lumbering around looking for food, as though nothing has happened. Unfortunately, these look just like regular zombies, so the only way to be sure is to crush the skull and have it collapse completely. For most people, the effort is too much and zombies usually keep their craniums. The Guard maintains that no corpses are allowed into the city, even if their skulls have completely collapsed, as a safeguard.
"Runners" are a phenomenon in the zombie world. While most of their counterparts lumber around, Runners will sprint aimlessly until they find a food source. They can be dangerous to deal with on foot, but from the ramparts surrounding Metropolis, they are fun to bombard with shells.
While Metropolis has never had a problem with this type, some scholars suggest that unattended nuclear plants will inevitably blow up, and cause zombies in the area to carry nuclear radiation with them, endangering victims in yet another way. You aren't sure what The Guard would do if they ever had to face radiation as well as zombies.
Zombies have been witnessed eating wild animals, so it is obvious meat, not humans, is their preferred diet. However, there has never been witnesses of any zombie animals - something scholars ponder about.