The intimidating size of the whale shark is offset by its gentle demeanor, making swimming with these behemoths a surreal experience.

Classified as the largest fish alive today, these gigantic creatures lumber through the water without much to worry about. Unlike most shark species, whale sharks are filter feeders, meaning they eat plankton, tiny fish, krill and crab larvae.

Swimming with whale sharks is an exhilarating experience, but may become a much less common adventure than it already is. Whale sharks may not be listed as “endangered,” but the gentle giants’ numbers are dwindling. They are considered “vulnerable” by the International Union for Conservation of Nature. Despite their relative scarcity, whale sharks can be reliably found in a dozen or so warm coastal sites around the globe, where they gather seasonally to feed. One of these sites is off the coast of the Yucatan, where it’s possible to swim with whale sharks gathering for a feast. Here, in 2011, more than 400 individual whale sharks gathered to feed on the eggs of the little tunny, the most common tuna species in the Atlantic Ocean.

The sight of a single whale shark, let alone 400, is powerful enough to make a lasting impression.

If you want to swim with whale sharks in Mexico or anywhere else along the Caribbean coast, you will have to wait until next year. When September hits, whale sharks typically disappear into the deep blue depths, away from the surface. That is, until summer returns.

In the meantime, check out this awesome video of another close encounter with giant whales.