For decades, waste, both humans and factories has been dumped in the ocean, untreated, unfiltered. But the waste has mixed with chemicals from paper mills, fertilizers plants, and radiation from nuclear facilitates seeping into the ground water – all reaching the oceans.
The waste is sucking the oxygen out of the oceans, causing them to die, and with that, ushers in the death of mankind.
A team is awarded the right to use their methodology with the blessings and financing from the U.N. Those scientists will use genetically modified microbes (just like in the Gulf of Mexico BP Deep Water Horizon Spill) to clean up the waste.
The microbes do their job, consuming the waste, saving the oceans, but in the process the microbes mutate and grow parabolically, becoming huge lizards that feed on human waste.
When the oxygen level is restored and the waste cleaned up, the creatures can no longer survive at sea and so gravitate to land. There, they burrow into the sewage system, coming up out of the toilets to enter the bodies of people sitting on the john.
They eat through human bodies, coming out their throats, with the victim’s guts dangling from their mouths. The creatures are in every country, killing off populations. When people go outside to use the bathroom, the creatures come out of the soil for their dinner. Finally, scientists realize that the creatures cannot survive in high oxygen environments and the world comes together to institute strict laws about toxic emissions, which raises the oxygen content in the air and soil, killing off the creatures.