Unit 731 was a Japanese human experimentation unit in World War II that captured soldiers, then used them for biological and chemical warfare. (This is an example of things Humanity is capable of, the burden of manipulation, torture, and blood, and is far from limited to simply Japan.)

Unit 731 performed a multitude of experiments, contributing to a learning experience, yet using the most twisted and cruel methods.  Unit 731 actually started as an innocent health agency, focused on research and public health, before slowly becoming a unit based on weaponizing diseases. Unit 731 took a large number of human prisoners, as all countries did, but they built knowledge on limitless suffering. The unit was disbanded at the end of the war, but Japan had not fully confessed to the existence of Unit 731 until around the 1990s but refuses to disclose information on the matter, nor the experiments. (Members of Unit 731 have confessed to some of the things they did, and the regret they hold on their actions.)

Unit 731  had several groups of people for research, but one of those happened to be based on limb injuries, and a particular scientist in the group had actually taken a special interest in frostbite, so he would often submerge the arms of prisoners in water filled with ice until the limb was frozen completely solid. He would try ways to rapidly warm the limb to rid it of the frostbite. Sometimes he doused the limb in hot water before holding it close to a campfire, or he would simply leave the limb untreated overnight to see just how long blood would naturally thaw out the limb. In another instance of research, they would give disease to the prisoners and simply observe to understand the effects of disease and injury on an armed soldier. They tested bayonets, swords, knives, and even flamethrowers on the victims.