Thicket of Iron Bamboo

Kurogane no Chikurin (黒金の竹林) is the 3rd chapter of the 5th volume and the 16th chapter overall of the Attack on Titan: Before the Fall manga, written by Ryō Suzukaze and illustrated by Satoshi Shiki.

Overview
The next morning, Cardina realizes that most of his peers have not got any sleep, as they were kept up by Kuklo's alarming declaration the night before. Jorge plans on showing the group the iron bamboo before he departs. The group make their way across town as Xenophon informs the group about iron bamboo. They soon come across Military Police and asks for a permit to allow them to see the iron bamboo harvesting area. By doing so, Xenophon makes aliases for Kuklo and Cardina - "Klow" and "Carl", respectively.

Jorge states that the bamboo is immensely firm as it harvests metals from the soil and accumulates them in its growth, possibly forming a new sort of alloy. Kuklo acknowledges that even the "Device" is made of iron bamboo, as the material is dense enough to kill Titans. Kuklo and Cardina learn that the Titans have a weak spot, which is how the alloy could be used to their advantage. This encourages Cardina to join Kuklo on becoming a soldier, and it motivates Sharle to be of service as Xenophon's apprentice.

Summary
It is uncertain whether the extraordinary circumstances of Kuklo's birth or his encounter with the Titans were what resulted in his certainty that the Titans would one day breach the Walls again, but his prediction did indeed come true fifty-five years later, when a Colossus Titan appeared outside of Shiganshina District. Although anyone could have predicted that the Titans breaking through the Walls, they chose to push such thoughts down into their subconscious thoughts, and ignore them. However, the four people who heard Kuklo's prediction, Sharle Inocencio, Jorge Pikale, Cardina Baumeister, and Xenophon Harkimo, all felt the horror of the day that Kuklo referred to, and understood his need to fight the Titans.

The next day, Jorge laments that he was not able to leave at dawn, as he planned, and Cardina silently notes that Jorge and Xenophon also had trouble sleeping after listening to Kuklo. Before he leaves, Jorge requests that Xenophon lead the group to the town's supply of iron bamboo. As the group walks through the city, Kuklo and Cardina are left in awe of the machinery inside the city. As Xenophon explains what different machines are for, Jorge informs the two that all of the military's weapons are forged in the city. When Cardina asks, Xenophon confirms that the iron bamboo is also produced in the city.

Xenophon informs Jorge that he still has trouble expanding production of the Bamboo, and that soon all of the military's weapons and armor will soon be manufactured from new material, but he is interrupted by the arrival of a Military Police Brigade carriage. Xenophon strikes up a conversation with one of the carriage's inhabitants, but they are more interested in talking wit their former instructor Jorge. Jorge informs the Military Police members that he is showing Kuklo and Cardina around the city, and one of the Military Police officers, Jentsch Dafner, agrees to let them into the iron bamboo harvesting spot on Jorge's recommendation. As the Military Police depart, Jorge admits his unease in Xenophon talking to the Military Police, and Xenophon explains that he did so to guarantee the four of them access to iron bamboo.

As they enter the complex holding the iron bamboo, Cardina marvels at the high security, and Xenophon explains that the security is do to the fact that the complex is the only place where iron bamboo grows. Kuklo, Cardina, and Sharle are left in awe at the sight of the bamboo. As the three inspect the bamboo, Xenophon explains that the bamboo is a type of plant that absorbs metallic elements from the soil and, over the years, continues to store metal inside of itself until it becomes hard like iron. Jorge demonstrates this by slashing one of the bamboo stalks with a blade, failing to leave even a scratch.

When asked by Sharle, Xenophon confirms that he crafted her knife from the iron bamboo. Cardina asks why the Military's equipment is not made from the bamboo, and Jorge explains that the iron bamboo has only been observed growing in the Industrial City, meaning it can only be harvested in limited quantities. Xenophon explains that although the city has been investigating how to manufacture iron bamboo, they have not been able to figure out what causes the bamboo's toughness and pliability. Kuklo declares that all he needs is at least one sword made of iron bamboo, but Jorge and Xenophon explain that The Device also uses iron bamboo, as the bamboo is used to make the pressurized gas chamber which fires The Device's wires, while the fiber in the bamboo's leaves were used to make the wires themselves.

Hearing that these designs were Angel Aaltonen's idea, Kuklo asks how Angel was able to defeat a Titan with The Device, since any wounds the Titans sustain heal instantly. Jorge explains to Kuklo that The Device is meant to allow the user to reach a Titan's weak spot. Cardina and Kuklo are surprised to hear that Titans have a weakness, and Jorge explains that it was never made public knowledge due to a lack of testing. As Kuklo reconfirms his intentions to join the Survey Corps, Cardina announces that he intends to join also, and Sharle decides to strat training as a craftsman with Xenophon in order to help Kuklo and Cardina by supplying them with equipment.

As he listens, Jorge recalls one of the Survey Corps' expeditions fifteen years prior to find the Titans' weakness.