When you were looking at the image of the electrical waste dump earlier on, and we mentioned cell phones, you probably didn't think they would pose as much as a problem. After all, they're smaller, and therefore they have to take up less room, right? Wrong. Cellphones may be much smaller than computers and other electrical devices, but there are also much more of them.
As well as being more numerous, they become obsolete a lot faster. A good refrigerator will probably last you ten years or more; a computer maybe five. The average life of a cellphone is only one or two years before the progression of technology makes them obsolete. When they're no longer of use to you, they end up abandoned in giant 'seas' like this in the Congo. The Congo is and has been a war area for many years, where people fight over land and resources. The material and technologies in the phones are just one more thing for the warring factions to fight over, meaning there is both an environmental and human cost to disposing of them in this way.