Among the world’s rivers, the Amazon reigns with the heaviest crown.

It begins in Peru, less than 75 miles from the Pacific shore, among the tiny glacial streams that trickle through the Andes. Those creeks become a river, which joins a network of other capillaries draining more than 3 million square-miles of South American land—water from mountains, foothills, and the world’s largest rainforest uniting to form a monumental flow that thunders clear across the continent until it gushes into the Atlantic. When measured by discharge, it is the largest river in the world: Every day, one-fifth of all the water that flows from all Earth’s rivers into all Earth’s oceans does it here, as the Amazonian flume. Nutrients in the spill support oceanic algae blooms hundreds of miles from shore.

Now, researchers have added yet another jewel to the river’s crown. A team of Brazilian and American scientists have discovered a new sponge and coral reef more than 600 miles long (1,000 kilometers), located at the mouth of the Amazon River. The reef appears to sprawl across more than 3,600 square miles of ocean floor at the edge of the South American continental shelf, from the southern tip of French Guiana to Brazil’s Maranhão State.

The New York Times Op-Docs and Annapurna Pictures are presenting a virtual-reality film, "The Click Effect," about the free-diving researchers in this Opinion essay. To view it, download the NYT VR app on your mobile device.

I HELD MY BREATH AND SWAM DEEPER, 10, 20, 30 feet. I heard a thunderous crack, then another, so loud they vibrated my chest. Below my kicking feet, two sperm whales emerged from the shadows, each as long as a school bus.

The cracking was coming from the whales; it’s a form of sonar called echolocation that species of dolphins, whales and other cetaceans use to “see” underwater. With these vocalizations, called clicks, the whales were snapping three-dimensional images of my body, and those of my diving companions, from the inside out — scanning us to see if we were a threat, or if we were food.

GEMMA SMITH is grinning like a child on Christmas morning. “It could be anything!” she says as our boat speeds past the rugged grey cliffs of Antikythera, a tiny Greek island midway between the Peloponnese and Crete. We are here to explore one of the world’s most famous shipwrecks, where divers once found an anchient computer.

The day before, the team discovered part of a large object buried beneath a metre of sand; now they are back to find out what it is. After years of preparation, there’s a feeling that today is going to be big.

The ship that sank here was a hefty wooden vessel, sailing west from Asia Minor towards Rome when it smashed against the island’s cliffs in the 1st century BC. It was discovered in 1900 by sponge divers, who salvaged the site under the direction of Greek archaeologists: the first scientific investigation of a shipwreck. They found bronze and marble statues, gold jewellery, ornate furniture, and gorgeous ceramics and glassware. Most intriguing was an anchent geared device - the Antikythera mechanism. Now understood to have been a clockwork computer, it was used to predict and display the movements of the sun, moon and planets in the sky (see “The solar system in a box“). “It is a symbolic place,” says Theotokis Theodoulou, an archaeologist at Greece Ephorate of Underwater Antiquities. “This is the cradle of underwater archaeology.”

We’re a step closer to understanding the microbial community that inhabits the ocean – and it has some striking similarities to the community that lives inside our guts. The microbiome of the world’s biggest ecosystem and one of the smallest appear to function in surprisingly similar ways.

Microscopic plankton produce a large proportion of the oxygen in the atmosphere – amounting to half of all oxygen produced by photosynthesis – but we know very little about these organisms. The data collected by researchers aboard the schooner Tara will change that. Between 2009 and 2013, the ship sailed the world’s seas and oceans, collecting 35,000 plankton samples – both microbial and multicellular – from the upper layers of the water.

The first batch of the Tara studies is published today, and it reveals that planktonic marine life is far more diverse than anyone expected. For example, we already knew of about 4350 species of microalgae, 1350 species of protists and 5500 species of tiny animals, based on direct studies of their appearance. But the new genetic evidence suggests that there are probably three to eight times as many distinct species in each group as currently recognised.

IT’S a game of life. A mass of fish dart in and out of a cloud of sperm and eggs off the island of Palau in the western Pacific, creating the next generation.

“The existence of red snapper spawning aggregations in Palau has been known for some time, so I wondered why there weren’t many photographs of it,” says photographer Tony Wu. Once he got in the water with them he discovered one possible reason – the fish were really fast and he struggled to keep up.

Spawning aggregations are common but not well understood. Two spot red snapper (Lutjanus bohar) gather in huge numbers where the edge of the reef meets the open ocean. They produce vast clouds of sperm and eggs – enough that plenty get fertilised, despite predators devouring much of the nutritious mixture. A strong current pulls any surviving fertilised eggs out to sea where they grow into adults. Some will make it back to the reef and spawn themselves, starting the whole process all over again.

Those same strong currents also made it difficult to get the shot, says Wu. “Getting into the right place at the right time was a challenge. The flow of water over the reef was steady and unrelenting, but I was able to position myself so that the action came to me.”

The spawning took place at Shark City diving site, and Wu spotted a few blacktip sharks lurking on the lookout for an easy meal.