They have a spherical body with three wire-shaped cilia on the left and three on the right. Instead of walking on the ground, they float through the air. They are minuscule machines which use the convection of the air to generate their own power to gather data indefinitely. They use a quantum signal they produce to transit over a direct electron beam to pass information between them and have created a type of network.
At some point in the past, Kihara Yuiitsu developed the UL Exploder, a device which gathers and detonates the UNDER_LINE's nanodevices in a false dust explosion, creating a space which couldn't be monitored for a very short period of time. When she showed it to Kihara Noukan, he praised her skill but scolded her, telling her not to bully "him".
Toaru Majutsu no Index
Battle Royale Arc
- Main article: Battle Royale Arc
Kakine Teitoku steals the Tweezers in an attempt to get a hold of one of the UNDER_LINE. After stealing Tweezer, Girl in the Dress asks regarding the results of the analysis, to which Kakine laments on how the data in UNDER_LINE are not enough to compete with Aleister with equal footing.
Later, Tsuchimikado Motoharu shows the Tweezers that he had recovered from the confusion after Accelerator's fight with Kakine. Motoharu shows to them the information inside the UNDER_LINE. Seeing that it was all a bunch of information they were able to get on their own, and that it was just information the higher-ups are gathering to keep an eye GROUP's actions, they become dismissive of it. However, Motoharu reveals that there is one more, and says the last one the list of information displayed on the Tweezer's screen is the word DRAGON.
Acqua of the Back Arc
- Main article: Acqua of the Back Arc
Due to Acqua of the Back's attack, UNDER_LINE was severely damaged and is creating a burden on the entire network. Aleister isn't worried about this development, and is going through the information the network has gathered.
Shinyaku Toaru Majutsu no Index
Ichihanaran Festival Arc
- Main article: Ichihanaran Festival Arc
As Thor and Touma prepare to break through the Windowless Building, Thor explains that he couldn't reveal his plan in full before now due to the higher-ups having set up a surveillance network monitoring everything in the city, different from the surveillance satellites and that has not been revealed to the public. The text makes clear he is referring to UNDER_LINE. As such, Thor makes the both of them use one of the many secret bases that previously belonged to Freshmen, and made a blind spot in UNDER_LINE to carry out their operations secretly.
Mental Out Arc
- Main article: Mental Out Arc
After dealing with Shundou Toshizou, Kihara Noukan sends a communication request to the Windowless Building and then speaks to Aleister Crowley. He proceeds to give a report about having taken care of the last of Aleister's 72 requests in eliminating groups or persons that had grown during his absence and refused to regroup, though he comments that due to UNDER_LINE Aleister should already be aware of this.
- Main article: Salome Arc
- Toaru Majutsu no Index Light Novel Volume 15 Between the Lines 4
- Shinyaku Toaru Majutsu no Index Light Novel Volume 15 Chapter 5 Part 10
- Toaru Majutsu no Index Light Novel Volume 15 Epilogue
- Toaru Majutsu no Index Light Novel Volume 16 Epilogue
- Shinyaku Toaru Majutsu no Index Light Novel Volume 05 Chapter 3 Part 7
- Shinyaku Toaru Majutsu no Index Light Novel Volume 11 Epilogue