The fin is directly in front of me, perhaps 30 meters (100 feet) away as it breaks the surface of the water. Behind the fin, a tail moves slowly from side to side, propelling the shark forward. The boat I was in is a short distance away, and everyone on it who has decided not to jump off yet is shouting or pointing, looking at what is coming toward me. Maybe I should have stayed on board as well …

It was just after dawn when we boarded the small fishing boat and left Isla Holbox. We were headed out to where the Caribbean Sea and Gulf of Mexico meet.

Located northwest of Cancún, Isla Holbox is a 26-mile-long (42 km) Mexican island separated from the mainland by a shallow lagoon, just a 30-minute ride in a water taxi from Chiquila on the mainland.

The mixing currents in this area and the nutrients they bring cause plankton to bloom, turning the normally crystal-blue waters into a murky brown. These tiny life forms attract a variety of marine life. Hundreds of small rays swim around the shallows between the small lagoon islands, and frigate birds and pelicans perch in the trees while several flamingos search for food just off shore.

It’s difficult to appreciate the size of a whale shark until it’s right next to you.

On the horizon, a small wave approaches. As it gets closer, the bodies of a thousand sardines glisten in the morning sunlight. We don’t stay long here, however; we are in search of the biggest fish around.

There were no nets or rods on our fishing boat turned excursion craft. The local fishermen abandon their trade for a few months each year to provide transportation for curious tourists like us — six backpackers on holiday in the Yucatán.

The seasonal bloom of plankton attracts an elusive, yet incredible creature, and we catch our first glance less than an hour after leaving shore. The boat’s engine stops and I'm given the “all clear.” My partner is still struggling with her mask and snorkel, but I'm too impatient to wait, and dive in.

And here I am, by myself, with a fin getting closer and closer. With waves splashing on my face it is hard to take deep breaths and relax. I put my snorkel in and look under the surface to see what is ahead. The water is murky and visibility is not more than a few meters, but I know the shark must be very close by now. There are a few last moments of the water around me appearing empty before I see a huge mouth starting to open.

It is over a meter (3 feet) wide and just in front of my face. I can do nothing except stare at this huge fish as it sucks in whatever comes between its jaws and wonder if I will meet the same fate.

But the ocean giant dives slightly and swims underneath me. I watch as more than 12 meters (39 feet) of its rough, spotted body swims just below the surface. I have to move to avoid the huge tail fin as it slices through the water next to my side.

While the whale shark appeared at first to be moving slowly, its tail quickly disappears into the murky water, and I try to follow. Looking above the surface, and taking what seems to be the first breath in a long time, I can see its fin moving slowly away.