Why did 12 sperm whales end up on German and Dutch beaches last week? What caused them to die? An autopsy of the more than 10-meter-long (33-foot-long) carcasses is supposed to reveal answers.

For the autopsy, the whales were transported from the beach to mainland Germany. "Difficult and fascinating at the same time," Almut Kottwitz - the deputy environment minister of the German state of Lower Saxony - told DW. A female sperm whale weighs 15 metric tons (16.5 tons), a male up to 60 tonnes. Even though the beached whales were still quite young, they had to be moved with cranes.

First, the whale's skin is cut into strips and peeled from the body. Preparers removed its muscles and ligaments, and gathered up the innards. "It cut me to the quick to see how these beautiful animals were being skinned and torn apart," an observer of the procedure told DW.

One of the skeletons was brought to Giessen University. Visible here is the whale's jaw - the bones will be treated for display. Another skeleton will be exhibited at a marine museum in Stralsund.

Some of the beached whales seem to have been undernourished, the atopsies found. No wonder: sperm whales feed on giant squids. And these don't live in the North Sea. "Their stomachs and their bowels were entirely empty," Almut Kottwitz told DW, adding that undernourishment seems to be "at least one of the reasons why they died." When whales are hungry and weak, they can get more easily lost.

Others among the sperm whales, however, seem to have been well-fed before they ended up on the beach, Ursula Siebert of the University of Veterinary Medicine in Hannover told DW: "They still had undigested beaks of giant squids in their stomachs, and feces in their bowels." Once on the beach, their weight compresses their blood vessels and lungs, causing them to die.

Sperm whales are not made to live in the shallow North Sea. They mostly live thousands of meters under water. In the North Sea, their echolocation doesn't work correctly. "When they arrive at the North Sea, they don't have muchof a chance of finding their way out of it again," vet and zoologist Siebert said. Ultimately, they beach and die.

A week ago, more than 80 short-finned pilot whales got stranded on the beach of the Indian state of Tamil Nadu. Conservationists managed to push some animals back into the sea, but most of them died. Fabian Ritter of the Whale and Dolphin Conservation estimates that thousands of whales beach every year.

The more socially minded a whale species is, the higher the number of whales that beach from that group. The long-finned pilot whale is one of these species, Fabian Ritter says. These whales have a very strong connection within their family group, or pod. "When the leader is ill or out of sorts, and swims onto the beach, the other group members follow - out of loyalty."

Conservationists warn that underwater noise is dangerous for marine mammals. "Whales have a very sensitive sense of hearing and communicate via sound with each other," Fabian Ritter says. "Loud noise such as from military exercises disorients them." Especially beaked whales are known to beach more often when there is too much noise underwater, Rittter says.

Solar winds disturb the Earth's magnetic field. Whales might get confused and beach more frequently, researchers found. At the end of December 2015 were three major solar winds, physicist Klaus Vanselow of Kiel University told DW. This might have led the 12 sperm whales to take a wrong turn into the North Sea - and end in death on its beaches.

The Yangtze is Asia's longest river and the third-longest river in the world. It also tops the list of river systems through which the most plastic waste flows into the oceans, according to a recent study. The Yangtze flows into the East China Sea near Shanghai and is crucial to China's economy and ecology. The river basin is home to 480 million people — one-third of the country's population.

The Helmholtz Centre for Environmental Research found 90 percent of plastic flowing into oceans can be traced to 10 rivers. The Indus ranks second on the list. One of Asia's largest rivers, it flows through parts of India and Pakistan into the Arabian Sea, supporting millions of people. While much plastic enters rivers because of a lack of waste infrastructure, sewage systems contribute too.

Plastic can enter the food chain as fish and other marine and freshwater animals ingest it. The Yellow River, said to be the cradle of Chinese civilization, is third on the plastic-waste list but that's not the only environmental problem with which it contends. Pollution has rendered much of the river's water undrinkable. Around 30 percent of its fish species are believed to have disappeared too.

Another of China's rivers, the Hai, comes in at number 4. It connects two of China's most populous metropolitan areas, Tianjin and Beijing, before flowing into one of the world's busiest shipping lanes, the Bohai Sea. The 10 river systems share traits, says the study. One is that they are located in densely populated areas with a lack of waste infrastructure and little awareness of recycling.

Generally thought to be the world's longest river, the Nile flows through 11 countries before entering the Mediterranean Sea from Egypt. Some 360 million people live in the river basin where its waters support agriculture — the region's main economic activity. Irrigation and evaporation mean the river doesn't even reach the sea in dry periods. Still, it comes in at number five in the ranking.

The Ganges is central to Indian spiritual life and provides water to more than half a billion people. Sewage, agricultural and industrial waste have made it one of the world's most polluted rivers, as have the multitudes of plastic that end up in it. Cleaning up the waste — as students are doing in this picture — is important, but experts say we must produce less and stop pollution at the source.

Here, workers clear floating waste from China's notoriously dirty Pearl River, which enters the South China Sea between Hong Kong and Macau. Sewage and industrial waste flow into the river delta, keeping apace with the region's incredible rate of urban expansion. Since the late 1970s, the delta has transformed from a mainly agricultural and rural region to one of the world's largest urban areas.

It's not until they hit urban and industrial areas that rivers feel the worst effects of pollution. Still, according to recent studies, plastic debris is even being found in remote and "pristine" locations. The Amur River rises in the hills of northeastern China and forms much of the border between China's Heilongjiang province and Russia's Siberia before it snakes out to the Sea of Okhotsk.

The Niger is West Africa's main river, supporting over 100 million people and one of the planet's most lush ecosystems. It flows through five countries before entering the Atlantic Ocean from Nigeria. Plastic pollution aside, extensive dam construction is affecting water availability — and frequent oil spills in the Niger Delta have caused widespread water contamination.

Dams are having major ecological and social impacts on the Mekong too. Around 20 million people live in the Mekong Delta. Many are dependent on fishing and agriculture for survival. The river flows through six countries in Southeast Asia, including Vietnam and Laos, and is tenth on the list of river systems that carry most of the 8 million tons of plastic that are dumped into the seas each year.