UNDER LINE

UNDER_LINE (滞空回線 (アンダーライン) ) is a network of miniscule machines spread all over Academy City which serve as the sole direct line of information to the Windowless Building and Aleister Crowley.

Background
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 miniscule 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.

Battle Royale Arc
Kakine Teitoku stole the Tweezers in an attempt to get a hold of one of the UNDER_LINE.

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.

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
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.