Many things, including the following: Gas, factories, trash, cigarettes, and many more. I can explain them to the best of my thought. Gas, because it runs off cars, gas is very harmful to the air, and which is caused by land. Factories are just like gas, no need to re-explain myself. Trash. When we do trash, all we do is put it under the land to decompose. That's really bad for the soil, leaving us less available space to farm. Latly, cigarettes, they do gas and are left on ground.