Hula Hoop

Hula Hoop is the largest particle accelerator in the world which is situated inside Academy City. The giant ring-shaped particle accelerator follows the shape of the wall, and is built 200m under the ground. Its control center was the same, it is near the outer edges of the district that bordered the outer walls. To prevent the Hula Hoop from releasing gamma rays in case of accidents, there are protection walls the level of nuclear shelters built around it. According to the different stages in speed, Hula Hoop is split into first, second and third rings, and accelerates particles separately starting from the smallest one. From the configuration, the third ring cannot work at a low speed of 30% of the speed of light. It’s something that can only be used in experiments that force particles to go over 70% of the speed of light. It is capable of accelerating protons to 99.22% of the speed of light, and maintain that speed for up to 300 seconds. Surpassing the speed it was designed to hold, or running it longer than it was supposed to would both lead to Hula Hoop’s explosion; that meant enough gamma radiation to cover a third of Academy City would be released.

DRAGON(≠Angel) Arc
At the beginning of Volume 19 Spark Signal, having revolted against Academy City, carefully made a plan that included the seizing of Hula Hoop and put into it the majority of their efforts and resources; the demand that they made to Academy City was "disclose all of the information you possess about DRAGON". Before Spark Signal's attack of Hula Hoop, they kidnapped around 30 primary school students on an astronomy field trip, along with their teacher and driver as a ‘bargaining chip’; while threatening to detonating Hula Hoop that would had destroyed a significant part of the city and flooding much of the remaining portion with radiation. The higher ups wanted to eliminate them and end the threat. GROUP used an elevator in School District 23 to gain entry to it where they then waited ten minutes to strengthen the signal from the Misaka network to Accelerator's collar, he then effortlessly eliminated Spark Signal in 300 seconds while also saving the first hostage that set to be executed.