Re-8

One day a group of kids stayed after school to clean up the school. They prepared sponges and a mop. They had already cleaned almost all the classrooms; all that was left was to clean the basement. As they were cleaning the basement, they noticed that the water dries out very quickly on the floor and there is dust almost immediately. They decided there was nothing strange about that. When they finished cleaning, they wanted to get out of the basement, but they realized there was something heavy behind the door. When they were able to free themselves and open the door, they saw that the school was almost destroyed, trees had already begun to grow around it. What had happened?