In 2006, he made a research trip to China to look for the baiji, also known as the Yangtze River dolphin. He and his colleagues searched the legendary river’s entire 1,700-kilometer main channel, twice. They counted nearly 20,000 large shipping vessels and more than a thousand fishing boats, but no river dolphins. At the end of their quest, they declared that the species was likely extinct, a conclusion now widely supported. For all the terrible harm that humans have done to cetaceans, drastically reducing their abundance and range, the baiji was the first of them that we ever completely extinguished. It happened in the 21st century.
Source: wiktionary