Task: Question Answering
Input (English): Passage: The seabed contains enormous reserves of minerals which can be exploited by dredging. This has advantages over land-based mining in that equipment can be built at specialised shipyards and infrastructure costs are lower. Disadvantages include problems caused by waves and tides, the tendency for excavations to silt up and the washing away of spoil heaps. There is a risk of coastal erosion and environmental damage.
Seafloor massive sulphide deposits are potential sources of silver, gold, copper, lead and zinc and trace metals since their discovery in the 1960s. They form when geothermally heated water is emitted from deep sea hydrothermal vents known as "black smokers". The ores are of high quality but prohibitively costly to extract. Small scale mining of the deep sea floor is being developed off the coast of Papua New Guinea using robotic techniques, but the obstacles are formidable.There are large deposits of petroleum, as oil and natural gas, in rocks beneath the seabed. Offshore platforms and drilling rigs extract the oil or gas and store it for transport to land. Offshore oil and gas production can be difficult due to the remote, harsh environment. Drilling for oil in the sea has environmental impacts. Animals may be disorientated by seismic waves used to locate deposits, probably causing the beaching of whales. Toxic substances such as mercury, lead and arsenic may be released. The infrastructure may cause damage, and oil may be spilt.Large quantities of methane clathrate exist on the seabed and in ocean sediment at a temperature of around 2 °C (36 °F) and these are of interest as a potential energy source. Some estimates put the amount available at between one and 5 million cubic kilometres (0.24 to 1.2 million cubic miles). Also on the seabed are manganese nodules formed of layers of iron, manganese and other hydroxides around a core. In the Pacific these may cover up to 30 percent of the deep ocean floor. The minerals precipitate from seawater and grow very slowly. Their commercial extraction for nickel was investigated in the 1970s but abandoned in favour of more convenient sources. In suitable locations, diamonds are gathered from the seafloor using suction hoses to bring gravel ashore. In deeper waters, mobile seafloor crawlers are used and the deposits are pumped to a vessel above. In Namibia, more diamonds are now collected from marine sources than by conventional methods on land. 
Question: Which ores are of high quality but prohibitively costly to extract?
Output (English): silver.