In the year 2029, a massive land sinkage occurs in the Mediterranean Coast. All oxygen in a 3 kilometer radius disappears from the Earth's crust.
Since that event, many chemical elements, such as oxygen, carbon, gold, molybdenum, and cobalt have disappeared from the face of the Earth.
The disappearance of the chemical elements has caused other things to vanish; forests, fields, buildings, ancient ruins, and even life itself.
Humanity soon fully realizes that everything is made from the chemical elements.
60 years later, the human population has been reduced to 1/10 of what is was during its peak period. After much research and investigation, the cause of the disappearance of the chemical elements is becoming clear. The chemical elements have been flowing to Nega Earth, another "Earth" which is located beyond the dimensional barrier.
A team called "Element Hunter" is formed to prevent the death of the Earth. All of the members are children who are 13 years old or younger. Only children, whose thought processes are more fluid than adults, can gain access to Nega Earth.

What subgroup did this? The subs are hilarious.