Graviton bombings

The Graviton bombings (虚空爆破事件) were a series of bomb attacks that occurred during the summer season in Academy City. It was featured in the Toaru Kagaku no Railgun manga, later adapted as a part of the anime's first season.

Perpetrated by Kaitabi Hatsuya, a level 2 esper and Level Upper user, using aluminum cans as explosive devices which he hides in mundane and seemingly harmless objects to target members of Judgment. Part of the Level Upper Arc, the graviton bombings displayed the tremendous power and danger of an unaccounted esper who has gained abilities using the Level Upper.

Kaitabi Hatsuya
Kaitabi Hatsuya is a level 2 ability user with the ability known as, Synchrotron. His ability allows him to use aluminum as base for accelerating gravitons, which in turns explodes shortly after acceleration.

Motivation
Hatsuya has a history of abuse by his peers, he has fairly weak body, easily submits to abuse, and does not fight back, making a good target for more attacks by his peers and other delinquents. However, he does not ignore his constant abuse by people, and blames Judgment and it's members for it, he sees Judgment as something that should protect the weak. He blames Judgment for his abuse, as according to Hatsuya is not doing their jobs properly and arrives late when he needs them the most. He enacts his vengeance on Judgment after acquiring the Level Upper.

Bombs
Hatsuya originally used aluminum cans as bombs where he puts them in waste bins. Hatsuya later uses a variety of harmless objects, such as dolls and bags to hide his bombs, which makes use of the aluminum in silverware where he keeps a sufficient supply in his satchel.

Failure of the authorities
In the Toaru Kagaku no Railgun anime, Hatsuya is able to cause several explosions before the authorities could find data on his abilities. This can be attributed to Judgment's own incapability to detect that they were the ones being targeted, and could not find links and patterns on the random bombings that occurred in the city, they also considered the possibility that the bombings were unrelated and was a copycat crime.

Failed explosion and the Graviton Bomber
In the anime adaptation, one bomb placed on some bushes in a crowded area failed to deliver an explosion massive enough to cause damage. This mistake gave the authorities a clue on who the likely perpetrator was, as a satellite has picked up a sudden acceleration of gravitons during the explosion.

Now knowing the nature of the ability narrowed down the suspect to a level 4 ability user, Kushiro Katabira, as she was the only ability user powerful enough to cause the explosions, as she was in a mysterious comatose state before the bombings occurred.

By this time, the knowledge of an ability user who causes explosions using gravitons have become widespread.

Graviton bombings continue
Hatsuya then targeted Green Mart, a local convenience store in Academy City for his next attack. Green Mart was detected by a satellite to be a location of a sudden acceleration of gravitons, a sign of an impending explosion. Judgment responded and tried to search for the bomb, but was too late, the bomb explodes and severely injures a Judgment member after protecting a girl from the explosion.

The Green Mart explosion, made the total count of Judgment members injured during the graviton bombings to nine. Prompting Judgment to have a meeting concerning the incidents, they analyzed the evidence discovered thus far using psychometry, but are unable to achieve any results. Judgment also cooperated with Anti-Skill in order to protect the citizens and to minimize victims, and also used all their resources in solving the case.

End of the bombings
Hatsuya is continued to be abused by delinquents and also his schoolmates during the height of the Graviton bombings and promised to himself that he will take care of his problems from now on, not needing Judgment anymore. After seeing Uiharu Kazari, a member of Judgment go into the Seventh Mist mall along with her friends Saten Ruiko, and Misaka Mikoto, Hatsuya decided to make the shopping mall as his next target. Carrying a frog doll with him, he inserted an aluminum object into it and accelerated it's gravitons, he then instructed a young girl to give it to Kazari.

However, his actions where not unnoticed, as his accelerating of gravitons were detected by Judgment. Uiharu Kazari, after hearing of a bomb plot in the mall from Shirai Kuroko, decided to evacuate the mall with the help of Mikoto Misaka. She is later called by Kuroko again, who tells her that she is the target of the next graviton bomb, sure enough the little girl complied with Hatsuya's instruction and tried to give the bomb to Kazari. However, she quickly realized it was a bomb about to detonate, Kazari threw it away and tried to shield the girl from the explosion. Mikoto however, tried to blow the bomb away with her signature move, the railgun, but fumbles her coin. The bomb then explodes, and would have engulfed them had it not been the efforts of Kamijou Touma.

After seeing that the explosion was successful, Hatsuya tried to leave the scene of the crime, however he was followed by Mikoto who confirmed as the Graviton Bomber. Hatsuya tried to fight back but is easily beaten by Mikoto Misaka, Hatsuya is later punched by Mikoto after using power as an excuse to do the things he did. He is later taken into custody by Kuroko.

Aftermath
The bombings forced Judgment to investigate the recent influx of espers who have suddenly gained powers in a short period of time and the inconsistency of esper data banks. They are later linked with the sudden increase of comatose espers, which they discover to have fallen on Hatsuya as well, and is later discovered that it was of the Level Upper's doing.