I believe the answer is: wells drilled on the ocean floor
Petroleum was created from the fossils of ancient organisms that has been dead and buried since million years ago. To extract it, we need to the ground till a certain depth until the oil leaked.
But, many companies decided to overlooked standard of safety in order to extract more oil from the ground. This caused a lot of oil to be leaked and absorbed to many water resources nearby, making it no longer consumable.