The current cetaceans, including the largest extant cetacean, the blue whale, are descendants of three families of cetaceans that flourished in the Miocene and Pliocene: the Mysticeti, the Odontocet

In this scenario, our modern oceans are a blend of ancient behemoths and present-day cetaceans. The Miocene and Pliocene cetaceans have not only survived but evolved alongside their modern counterparts, creating an underwater world of epic proportions. The ocean's apex predator, once the Livyatan with its immense size and gargantuan teeth, has now adapted to be faster and more efficient in hunting due to the competition with modern orcas and great white sharks. It retains its size but boasts a leaner physique, reflecting the need for speed in the contemporary, more dynamic marine environment. Acrophyseter, initially medium-sized predatory sperm whales, have morphed into deep-sea specialists, exploiting niches currently occupied by the giant squid. Their eyes have further developed to focus on bioluminescent prey in the utter darkness of the ocean depths, a feature they share with modern sperm whales. The long-snouted Eurhinodelphis has taken to tropical shorelines, where they become efficient hunters of near-shore and reef systems, capitalising on an ecological niche somewhat akin to our present-day bottlenose dolphins. Squalodon, once toothed whales similar to today’s dolphins, have grown larger to contend with modern humpback and gray whales for feeding areas. Remarkably, Macrokentriodon has adapted to become freshwater dwellers, much like today's river dolphins, feeding on rich fish populations in river estuaries. Australodelphis, with its streamlined body and robust tail, has transformed into the cheetahs of the sea, chasing down speedy fish and squid with short bursts of tremendous speed. Similarly, Hemisyntrachelus traded size for agility, evolving as nimble hunters in the open ocean, somewhat resembling today’s common dolphins. The rare Platalearostrum and Kentriodon, once small to medium-sized dolphins, have found their place in deeper waters, diving to great depths to feed on squid and fish species not accessible to other cetaceans, operating similarly to modern beaked whales. Bohaskaia has taken a unique path, developing specialized teeth to crack open the shells of crustaceans off the ocean floor, similarly to the modern day walrus. Odobenocetops, which was already unique in its asymmetrical tusks, adapted to colder Antarctic waters, using its tusks to break through ice and access food sources below, behavior shared with present-day narwhals. Eobalaenoptera evolved to exploit the flourishing krill populations in Antarctic waters, along with Balaenoptera bertae, having a similar filter-feeding lifestyle like our modern blue whales. Cetotherium, Herpetocetus, and Piscobalaena, once thought to be ‘dwarf’ baleen whales, have since evolved into giants of the species, using their immense size to deter predators and to access deeper feeding grounds, matching and sometimes surpassing modern rorquals in size. Semirostrum and Parapontoporia, both long-beaked dolphins, have evolved strategies for digging in the seafloor for food, almost like modern sea otters and walruses. The survival of these ancient creatures into the modern era has created an unparalleled oceanic diversity, a thriving world beneath the waves where both age-old giants and modern marvels coexist and continually adapt for survival.