The genetic variation present in modern populations stores a wealth of information about their past history. For baleen whales, this new genetic detective tool can help uncover the population size and growth potential of whales before whaling began. The first results of this approach have shown that whales have too much stored genetic variation given the small numbers of animals traditionally imagined for pre-whaling oceans. Instead of 100,000 humpback whales in the pre-whaling world, genetic estimates suggest over 1,000,000 animals were the typical global humpback census. Similarly, genetic variation of fin and minke whales is far higher than expected. The deepest dish of whale diversity has been sampled from the meat markets of Japan. Southern Ocean minke whales hunted under scientific permit show more genetic diversity within this population than any whale population yet measured, suggesting a typical population size of over a million individuals for this one species in Antarctic seas. To date, these estimates are based on limited data and require careful confirmation with multiple genetic tools. The promise of this approach is that the whale genome represents a logbook of the history of these threatened populations. When we fully learn to read this logbook, we will be poised to understand the history of the world’s largest animals and their role in the ecology of the oceans.