The Raptor Pen, also known as the Raptor Pit, or Holding Pen, was a huge reinforced cage on Isla Nublar where the park's Velociraptors were held under maximum security. The Velociraptors originally had a Paddock on the island like any other animal, however The Big One had the pride attack the fences when the feeders came. While their paddock was under construction, this was their temporary paddock as it was too small.
It was first seen at the beginning of the film when the Jurassic Park security team were trying to get the alpha-female, "the Big One", into the pen, but a worker named Jophery is killed in the process.
The Containment Unit was seen again when on the tour by Alan Grant, John Hammond, Ian Malcolm, Ellie Sattler and Donald Gennaro. They witnessed a 300-pound cow being lowered into the cage. The cow was quickly attacked and eaten by the three Raptors.
The Containment Unit was last seen when Robert Muldoon and Ellie Sattler discovered that the electrical fencing around the pen had been broken through and that the three Raptors had escaped after the fence's had been shut down.
In the novel, the Holding Pen was first seen when the endorsement team, specifically Alan Grant, wanted to see the Velociraptors. Ed Regis and Henry Wu told Grant that the Velociraptors hadn't been implemented into the park setting yet, for security reasons. The raptors had escaped their original enclosure several times, and had killed 2 workmen, and injured a third. After attempting to use radio collars on them (they quickly chewed them off), it was decided that they be put in a more secure paddock. The Holding Pen consisted of a strong, 12-foot tall electrified chain-link fence with barbed-wire at the top. It was located a short distance from the Visitor Center in the main Visitor Compound.
It is also seen in Jurassic Park River Adventure.
The Hatcheries in Jurassic Park: Operation Genesis are modeled after this pen.