The Hollows is an area in Paragon City just east of Atlas Park. It was formerly the suburban area known as Eastgate before the Trolls destroyed it in 2000 with a series of tunnels that resulted in the world's largest sinkhole. Although the access gates in Atlas Park and Skyway City say "Eastgate", most citizens refer to it as "The Hollows".
"The Hollows" are the first in a series of Hazard Zones for registered heroes to visit, and many contacts were notorious for sending heroes there just for the sole purpose of being there.
Once, Eastgate was a beautiful suburban part of Paragon City, with its shopping centers and central mall. Unlike Woodvale, Eastgate prided itself with middle-to-upper-class tastes. Luxury condominiums and McMansions became the norm for many parts of the area.
But lurking literally underneath the area were the Trolls. Fueled and mutated by the illegal substance Superadine, the Trolls believed that these hilly lands belonged to them, and thus, in 2000, began to create caverns for them to prepare to take over the area.
They had no idea that the explosives they were planting were above even older caverns which, when detonated, swallowed much of the area into the ground. This became known as "The Hollowing".
Today, the Trolls lay claim to much of the area, but they are not the only ones that do.
(For more information, including in-game details, visit the Paragon Wiki entry.)