If an epic underwater encounter with one animal is good, then these six aggregation scuba diving experiences are sure to stick with you for a lifetime.

It’s only natural that as we progress as divers, so too does the scale of what impresses us. Most of us can recall when all it took was the sight of one triangularly shaped dorsal fin nearing. But a solitary shark, although always transfixing, loses a bit of its thrill factor with time. And so we begin to court numbers. And not just because there’s something so visually arresting about letting dozens, if not hundreds, of an animal lull us into admiration. In swimming amid these schools, pods, colonies and shoals, the chances for an intimate shared moment become so much bigger, and in turn, so does the connection we feel not just with the animal, but also with something infinitely wilder and more powerful.

READ MORE: The Planet's Best Big Animal Dives

Sardine Run | SOUTH AFRICA

South Africa’s annual sardine run — dubbed the greatest shoal on Earth — is not for the faint of heart. Nothing about this dive is typical. There is no fixed point of reference: Divers drop into an ever-shifting swirl of fins amid blue water. There is no set dive time. Instead, everyone — hunters and spectators — stays until a baitball is swept up into maws, which could take over an hour.

“What I found challenging about the situation is … everything you can imagine,” says Doug Perrine, an underwater photographer who’s dropped in on several sardine runs. You can’t pay attention to the baitball or a shark will likely bump you.”

Rather, divers must balance focus, readying for potential impact from every direction. But as much as that nonstop action is physically taxing to watch — while staying out of the way in a bullsin- Pamplona manner — it’s also what makes it beyond memorable.

Think of it this way: In the sharkdiving arena, the sardine run is the grand finale of any Fourth of July fireworks show: frenetic and overwhelming, but sensationally pleasing for onlookers in the grandest possible way. peri-peridivers.com

In Season

• Every May to July, an estimated 30,000 tons of sardines blacken the Indian Ocean roughly 100 miles off the city of East London, South Africa, attracting with it legions of sharks, impressive in number and variety. For this seemingly endless buffet of fish sticks, nearly every shark species in the area — bulls, duskies, blacktips, bronze whalers, great whites and hammerheads — arrives by the hundreds, along with many other predators.

Whale Sharks | ISLA MUJERES, MEXICO

The waters off Isla Mujeres, Mexico, hold the record for the most whale sharks ever recorded in an aerial survey: A mass of 420 swarmed this satellite island of Cancun on August 9, 2009.

You can thank the Yucatan shelf’s bathymetry for attracting the giant fish. Dr. Rachel Graham, who’s dedicated to the protection of marine megafauna as executive director of the MarAlliance organization, explains: “The current that becomes the Gulf Stream has to funnel through the relatively narrow corridor between the Yucatan and Cuba, and in doing so, passes through deep water, pulling up tons of nutrients onto the Yucatan shelf, creating an upwelling. And that creates planktonic blooms.” That plankton feeds the zooplankton and triggers bonito spawns, which the whale sharks scoop up.

And that is the behavior you’ll see off Isla Mujeres: lots and lots of eating.

“They’re feeding — they don’t care about people and will mow you down,” says Graham, although it’s recommended that snorkelers avoid lingering in front of the sharks’ mouths because that startles them. “Get too close, and it’ll close its mouth and swim away. Imagine being in a restaurant and every time you lifted a fork to your mouth, people shouted, ‘Hey, hey!’ or took your photo.”

Respectful onlookers are in for an unforgettable experience.

“One of the cool feeding mechanisms is bottle-style feeding,” says Graham. “They will sit vertically and gulp the dense patches of eggs. The amount that they can sieve through their gill rakers is unbelievable.” prodivemex.com

In Season

• Every May to September, whale sharks gather off Isla Mujeres without fail. Snorkelers who keep a distance of 10 feet or more enjoy longer encounters, and perhaps witness more interesting behaviors.

Humpback Whales | THE SILVER BANK

The Silver Bank, essentially a 100-foot-deep sandbar 75 miles north of the Dominican Republic, is home to the largest breeding aggregation of humpback whales in North America.

Here, the courtship action is nonstop.

“A lot of the surface behaviors we see — lob-tailing, pectoral-fin slapping, breaching — really, all of the big, splashy surface behaviors are all done to attract a mate or intimidate a competitor,” says Capt. Gene Flipse, founder of Conscious Breath Adventures, which charters the Belize Aggressor IV liveaboard dive vessel. What’s most common to see are groups of males competing.

“They’re charging around, and it’s a high-energy, very thrilling experience to witness hundreds of thousands of pounds of testosterone physically fighting for a female, although we don’t swim with those whales,” says Flipse. “For us to swim with them, they need to be not swimming at all, which means resting.”

Resting need not mean sleeping. The mothers will sometimes be stationary while nursing, although sometimes they swim at a slower pace.

“It’s almost like watching an airplane refuel in midflight as the calf maneuvers into position while the mother moves very slowly,” says Flipse. “You feel privileged that the mother whale is comfortable enough to share this intimate experience with you.” aggressor.com

In Season

• The Sanctuary of Marine Mammals in the Dominican Republic offers permits to just three boats, allowing roughly 600 people to visit each year. Belize Aggressor IV visits mid-January to early April.

Manta Rays | INDONESIA

The complex currents flowing around Indonesia’s 15,000-plus islands help make the destination the world’s second richest in biodiversity. They also support a year-round population of mantas — both the reef manta ray, preferring more-shallow water and growing up to 18 feet across, and the giant oceanic manta ray, whose up-to-23-foot wingspan can dwarf a diver. Because of the abundance of rays here — especially concentrated around the islands of Komodo, Nusa Penida and Raja Ampat — researchers have focused on these reefs, pinpointing which sites are feeding areas and which are cleaning stations.

Moreover, says Sarah Lewis, project leader for Manta Trust’s Indonesia team, “There are so many well-known manta sites where the manta rays are very used to dive tourists and thus often get very close, making for brilliant encounters. We don’t typically associate fish with intelligence or showing signs of curiosity toward divers, but mantas are different. They have the largest brain-to-body ratio of any known fish, and with this intelligence comes curiosity.

“Having a wild manta go out of its way to approach, hover next to you, and look you in the eye is an incredible experience and a fascinating behavior,” Lewis says.

Because it’s common to see a dozen or more mantas, divers will start to recognize behaviors, such as mating rituals. “At the cleaning stations, the female will be followed by a train of male mantas,” says Lewis. “She leads them on a fast-paced chase, twisting and turning throughout the water column. This is one of the most exciting and beautiful manta behaviors to witness.” thearenui.com

In Season

• Mantas can be seen off Indonesia year-round, but from December through April, the Arenui liveaboard focuses on Raja Ampat, where aggregating mantas can be seen with consistency at cleaning stations.

Gray Seals | FARNE ISLANDS, U.K.

The Farne Islands shelter the biggest gray seal colony in the U.K., but if that isn’t reason enough to travel to this North Sea location east of Edinburgh, Scotland, consider this: Gray seals aren’t nearly as aggressive and overbearing as the Pacific harbor seals that divers who visit California know all too well.

Says Alex Mustard, marine biologist who’s been nibbled on many times by both varieties: “The gray seals are just as friendly but more chilled out. They do everything at a much more relaxing pace. In much of their range, they are wary and keep their distance, but in the Farne Islands, they are very used to divers, so they’re usually very friendly.”

That friendliness looks like them grabbing a hold of ropes, nibbling dive hoods and tugging on fins. A colony estimated at around 5,000 seals populates the Farne Islands, and when the playful seals aren’t swimming about in the water, you can see them resting on land.

“The younger the seal, the more curious it is,” says Steve Gibson, director of Sunderland Scuba Center, which organizes dive trips to the Farne Islands.

The seals don’t migrate much throughout the year, occasionally moving to the nearby Shetland and Orkney islands, but for the most part, they stay put, moving mainly in search of a lee when wind direction changes, ensuring that divers will have reliable encounters.

However, water conditions are less dependable, which is why the dive season typically extends April through November, when the North Atlantic waters are as calm as the attitudes of the seals. sunderlandscubacentre.com

In Season

• “We prefer to dive with them in the pupping season, August to October,” says Steve Gibson of Sunderland Scuba Center. The National Trust recorded 1,876 pups born during the season in 2015.

Hammerhead Sharks | THE HAMMERHEAD TRIANGLE

No other patch of ocean in the world sees greater numbers of hammerhead sharks than the triangle between the Galapagos Islands of Ecuador, Cocos Island in Costa Rica, and Colombia’s Malpelo Island.

Here, divers find themselves in dark waters as schools of 300 swim overhead, crowding out all sunlight.

Moreover, it’s not just the size of the schools that overwhelms, but also the proximity with which these sharks allow divers, thanks largely to healthy local populations of king angels and barberfish, which groom them at cleaning stations.

“If every time you breathe, there is a big blast of air, you’ll freak them out, but otherwise, a hammerhead will come within inches of you,” says Wayne Hasson, president of the Aggressor Fleet, which visits Cocos and the Galapagos.

To locate the biggest aggregations, it’s not seasonality that determines their whereabouts, but thermoclines. Hammerheads are attracted to these meetings of cold and warmer water, although tracking where the temps will mix isn’t exactly easy. Says Hasson, “The currents here always travel in the same direction, but the upwellings and downwellings can change from one week, one month to the next.”

The good news is, regardless of currents, hammerheads stay within this triangle year-round. “I’ve been to these islands every month of the year,” Hasson says. “For seeing hammerheads, it’s always a good time to go.” aggressor.com

In Season

• It’s not seasonality that determines the hammers’ locations, but thermoclines.

Honorable Mentions

Bumphead Parrotfish

• These fish will sometimes bump coral headfirst to break it into smaller pieces for consumption.

Because fish don’t wear watches, they rely on moon phases to serve as a clock. In Palau, an island nation north of Indonesia, the new moon serves as a meeting-up time for thousands of single-and-ready- to-mingle bumphead parrotfish. The best part: This event happens during a four-day window every month.

Goliath Grouper

• Dominance-asserting males emit a resounding pop. It’s a given that a September dive in the Atlantic off the town of Jupiter, Florida, will reveal more goliath grouper than you’ve ever seen on one tank. What isn’t known yet is the precise moment the water column fogs with their spawning. But until science pins down the big nighttime event, it’s worth a trip to see these fish on overdrive.

Lemon Sharks

• The waters off the town of Jupiter also serve as a spawning site for lemon sharks. From December to the full moon in March, as many as 50 of these rather docile sharks can be seen during a single daytime dive as they warm up to mate. As for why they choose the well-traveled, it could be to ensure genetic diversity, or perhaps out-of-towners talk a sweeter game.