Fifty years ago, the River Thames was declared biologically extinct; it was so polluted that no organism could survive in it. But now, sightings of marine mammals in the river are becoming more and more common.

In the last 10 years, the Zoological Society of London (ZSL) has received 2,732 reports of whale, seal, porpoise and dolphin sightings in the River Thames. Seals are often seen along the Canary Wharf, a major business district in London, BBC reports.

"Many people looking into the Thames see a murky, dirty environment," ZSL European conservation projects manager Joanna Barker said. "But, actually, beneath the waves, it is full of life. We have a huge range of fish and invertebrates, and also top predators," she told BBC.

Barker said the presence of marine mammals in the river indicates that the water has become cleaner and that fish species have begun to thrive in it again.

"As a top predator, their presence is a good sign that the Thames is getting cleaner and supporting many fish species. The presence of these animals is also a great example of how urban environments are important for wildlife," she said, according to Mashable.

Seals have been spotted upstream as far as Teddington and Hampton Court Palace, while porpoises and dolphins have been spotted as far as Teddington Lock.

For Barker, this indicates that the area has become an important environment for the conservation of these animals. Unlike the North Sea, the Thames Estuary is more sheltered and provides a range of habitats for marine mammals.

However, ZSL said the marine mammals continue to face the threat of pollution, loss of habitat and dangerous encounters with boats, CNN reports.

The ZSL has advised the public to keep reporting sightings of marine mammals along the Thames. It is conducting a survey of seal populations in the area.

Meanwhile, people are sharing pictures of Thames marine mammal sightings on Twitter.

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