There is nothing quite like swimming in tandem with a gentle sea creature almost four times your size.

Snorkling is usually a solitary, calming ocean sport. You gently paddle where you want to go, letting the currents carry you over what you want to see. When an unexpected 20-foot whale shark crossed my line of vision while snorkeling recently, I literally gasped underwater.

I raised my head to check my bearings and clear my snorkel. Then I started swimming. I swam faster than I had in all my life.

Delighted to be wearing a top-quality pair of strong fins, I was able to keep up with the seemingly mythical creature next to me. Nearly matching him stroke for stroke, I wanted to reach out and touch his glistening, bumpy skin. I didn’t, because near the Isla de Mujeres off the coast of Cancun, Mexico, whale sharks are fiercely protected.

Whale sharks are filter feeding carpet sharks, the largest species of any fish. Unlike whales, sharks are not mammals. The whale shark carries its name due to its massive size, which can be as long as 40 feet.

They pose no predatory threat as their diets are composed of plankton and small fish. That isn’t to say you shouldn’t beware of these creatures. His powerful tail, the one our boat captain warned us to beware of, slapped my son on the shoulder and he felt it!

The whale shark’s skin general is dark gray, with a white belly. It is polka-dotted, with pale gray or white spots and stripes that serve as unique identifiers. The fish has five sets of working gills.

Frankly, it was the whale shark’s head that intrigued me most.

Unlike most of its cousins, the whale shark has a round, flat head. The mouth is at the front of the face, instead of underneath.

Would I have been so calm watching that yawning mouth come toward me if we had not just been educated about their diet? Inside the nearly five-foot-wide mouth are as many as 350 rows of tiny teeth, constantly filtering tiny particles of food.

In Cancun, you can snorkel with the whale sharks for a fee. My children and two of their friends joined me in a recent, exhilarating excursion.

The whales come to the same spot about an hour off the coast every single year. How they know they are home in the middle of the ocean, I’ll never know. They come for the abundant krill in the warm waters, then move on to the next destination to mate or give birth.

As a scuba diver for more than 30 years, I long for underwater encounters like these. After 100s of dives, my stories revolve around a handful of favorites: the time I gingerly held a pregnant male seahorse; the cave that became a nightmare as 10 divers crowded in and panicked, stirring up silt; the lurking sharks circling us in the Blue Hole; the whirling dervish of barracuda spinning like a cyclone in Malaysia.

Now, I add the whale sharks of Mexico to the list. Technically, they don’t belong on my diving experience tally, but what the heck. I didn’t even have to strap on my  weighty gear or sink far below the surface to be thrilled.

And what a thrill it was.