Let's go with the blue whale this time:

It always struck me that modern animals in general seem much smaller than their prehistoric ancestors. The megalodon dwarfed the Great White Shark, titanoboa was far larger than the largest of anacondas, and the indricotherium, the largest land mammal ever to have lived, went extinct millions of years ago. The biggest exception (literally) to this trend is the blue whale, the largest animal to have ever existed at any point in history. Blue whales get to be around 98 feet (or 30 meters) long, and weighs about 180 tons. The heart alone is absolutely massive.

The blue whale's vocalizations make it the loudest as well as the largest animal: It can produce up to 180 decibels worth of sound, but the frequencies at which it produces these sounds is often well below what the human ear can detect. Like many other things about the blue whale, the purpose of the vocalizations is currently unknown. Finding out may be an uphill battle, as they are currently an endangered species. An international ban on hunting blue whales was imposed in 1967, but prior to then whaling was so rapant as to wipe out 99% of the population of the species. In 1931 alone, 29,000 blue whales were hunted and killed, which is larger than the current extant population (around 10-25,000).