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Scientists from the University of Liverpool believe they have discovered one of the main reasons why marine mammals are able to hold their breath up to an hour.  It all depends on myoglobin, which is an oxygen-storing protein found in all mammal's muscles.  

Dr. Michael Berenbrink of the Institute of Integrative Biology at the University of Liverpool said that scientists had known for some time that marine mammals manage to maintain very high levels of myoglobin in their bodies.  But as Berenbrink states, "at high enough concentrations, [proteins] tend to stick together, so we tried to understand how seals and whales evolved higher and higher concentrations of this protein in their muscles without a loss of function."  

In order to find out how the storage was possible, a team of scientists extracted and examined myoglobin from the muscles of a variety of mammals.  Their findings showed that mammals with a greater capacity for deep-diving had evolved a "non-stick" variety of myoglobin that has a subtle but vital change in its chemistry; it is positively charged.  The consequence of the proteins being positively charged is that they repel one another and avoid sticking together, thus allowing the animal to store much higher concentrations of myoglobin in the muscle tissue and thus more oxygen.

Not only does the discovery explain how modern-day marine mammals are able to hold their breath for so long, but it allows scientists to make relatively accurate assessments of the dive times of the ancestors of whales.

This finding also has implications for medical research, as copying this sort of "non-stick" myoglobin could be used towards developing oxygen-carrying liquids that would provide an emergency supply of oxygen when a blood transfusion is not possible.

For more information, read BBC News - Deep-diving mammals' secret revealed