A male great hammerhead shark swims in the Bahamas at sunset in this image captured by National Geographic photojournalist Brian Skerry.

For nearly 30 years, Skerry has been swimming with and photographing sharks, including great whites, tigers, bulls, blacktips, and great hammerheads all over the world. In his first blog post for the Smithsonian Ocean Portal, "Swimming with Sharks," Skerry reflects on these exhilarating experiences.

Scientists debate the purpose behind these sharks' hammer-shaped heads. A commonly accepted theory is that the shape allows the shark to scan a wider area of the ocean through its sensory organs. Of the eight species of hammerheads, the great hammerhead shark (Sphyrna mokarran) is the largest, reaching a maximum length of 6.1m (20 ft). Hammerheads are highly sought after for their fins, suffer a very high mortality rate from bycatch, and reproduce only every two years. The International Union for the Conservation of Nature lists the great hammerhead's status as "Endangered."