Solid waste in the oceans

Submarine junk
Undeveloped competence in maritime demolition and salvage
Offshore oil platforms are designed to stay in place, not to be dismantled when the oil runs out. In 1986, there were 6,000 oil platforms in the world's oceans; the greatest concentration is 4,000 in the Gulf of Mexico, whose removal cost of removal has been estimated at $2 billion in 1985, rising to $7.5 billion in 2020. The UK has the largest number of gigantic steel rigs, 139 in deep waters of the North Sea. Their removal is estimate at £6 billion. The largest platform weighs 40,000 tonnes, is anchored in almost 600 feet of inhospitable ocean and stands almost 1,000 feet from sea-bed to tip. The demolition exercise is untested and daunting. Because the legal position surrounding removal is fuzzy, it is likely that the technology will be fully developed only when requirements are clarified. The cheapest option is to leave them there with a beacon on top to warn shipping.
Aggravated by 
(E) Emanations of other problems