The deep ocean, below 2,000 meters, is crucial to the health of ecosystems above and below the surface, yet it remains one of the least-explored parts of our planet. At the same time, this remote and ever-changing volume plays an important role in regulating global climate, something that is especially important to understand as we look to the ocean for solutions to some of the generational environmental challenges we face. But crushing pressure, near-freezing temperatures, and permanent darkness make it difficult for humans and technology to get a look at what is going that far beneath the surface for any length of time. It is with the help of submersibles like those operated as part of the National Deep Submergence Facility (NDSF) at the Woods Hole Oceanographic Institution (WHOI) that researchers from around the world have been able to make important and exciting discoveries in recent years.

There are three broad categories of deep-submergence vehicles represented by each of the three vehicles in the NDSF fleet. The first and longest-serving are human-occupied vehicles (HOVs). HOV Alvin, which is owned by the US Navy and, like the rest of NDSF, funded by the National Science Foundation, has been exploring the ocean in ever-increasing depths since 1964. Because it is owned by the Navy, its design, operation, and maintenance must meet strict requirements established by the Naval Sea Systems Command (NAVSEA). The result has been a nearly flawless safety record over the course of its storied career, which includes more than 5,200 dives transporting more than 3,000 people to the depths. In 2022, Alvin was certified to dive to 6,500 meters—a little over 4 miles—extending its ability to support oceanographic research to roughly 98% of the global seafloor.

AUV Sentry's deployed from R/V Atlantis off the coast of Bermuda (Image credit: Lu Lamar)

Alvin has also captured public attention and imagination with several high-profile expeditions that speak to the wonder and mystery of the deep ocean. In 1977, scientists diving in Alvin at the mid-ocean ridge near the Galapagos Islands encountered warm, chemical-rich fluids flowing from the seafloor. They were also astounded to find lush ecosystems flourishing where biologists thought there should only be sparsely populated rock and sediment. The discovery of hydrothermal vents rewrote textbooks and reshaped our view of life on Earth, as well as our understanding of the potential for life in the Solar System. In 1986, just one year after a team from WHOI and the French National Institute for Ocean Science (IFREMER) discovered the wreck of the RMS Titanic, a WHOI-led team returned to explore and document the site with Alvin and the prototype remotely operated vehicle Jason Junior. Images from the wreck rekindled fascination with the great ship that continues today. Both of these highlight a singular strength of human-occupied submersibles that they are able to transport human eyes, minds, and curiosity to the seafloor to engage in firsthand exploration and discovery.

While it may be the most well-known, Alvin is certainly not the only deep-sea vehicle getting to the bottom of things, nor are human-occupied submersibles the only way to conduct advanced deep-sea research. For nearly 50 years, remotely operated vehicles (ROVs) have provided a reliable platform to survey, sample, and document the seafloor in a wide range of conditions and locations. ROV Jason (actually the second iteration of the vehicle), developed and operated by WHOI engineers, has explored hundreds of hydrothermal vents in the Atlantic, Pacific, and Indian Oceans. Like Alvin, Jason can dive to 6,500 meters while remaining connected to the surface by a reinforced fiber optic cable through which it receives power and command-and-control signals and returns data and ultra-high definition video to the surface. This gives it the ability to remain submerged for long periods of time and to get much closer to often explosive seafloor phenomena than Alvin’s safety tolerances might allow. In 2006, Jason captured a close-up view of a volcanic eruption 1,800 feet underwater south of Japan. The vehicle got so close that engineers had to pry solidified balls of once-molten sulfur from Jason’s frame when it returned to the surface.

ROV Jason is deployed from RV Sikuliaq during an Ocean Networks Canada Observatory expedition. (Image credit: Jeff McGuire)

Where humans often fall short, whether inside an HOV or at the controls of an ROV, are in situations that require constant vigilance or repeated but precise and consistent actions. That’s where the robots come in. Autonomous underwater vehicles (AUVs) are programmable platforms that carry a wide range of sensors and navigate the ocean using a variety of propulsion modes. Some AUVs are even capable of real-time decision-making and might alter their mission profile based on their surroundings, essentially mimicking a scientist’s capacity to follow an unexpected or fortuitous encounter. One of the most advanced AUVs available to the ocean science community is Sentry, which makes up the third of three NDSF vehicles.

Sentry has an extensive and flexible science sensor suite to map and survey the seafloor and water column in high resolution and has even been heavily modified to collect free-swimming planktonic larvae in the water column to study how otherwise sessile seafloor organisms are able to spread from one location to another. Sentry has also responded in times of critical need, such as mapping hydrocarbon plumes extending from the Deepwater Horizon well and helping recover the voyage data recorder from the sunken cargo ship El Faro.

Over the past sixty years, Alvin, Jason, and Sentry alone have made more than 8,000 dives, but much of the seafloor and even more of the volume of the ocean is yet to be explored. Deep submergence, whether for scientific research, deep-sea exploration and discovery, or even personal curiosity and tourism, each have their place. Regardless of the goal, opening the ocean’s secrets to examination ultimately makes us all better stewards of this sorely misnamed watery planet we call home.

To find out more about WHOI’s work in the deep sea, visit: https://www.whoi.edu/

This feature appeared in Environment, Coastal & Offshore (ECO) Magazine's 2023 Deep Dive III special edition Deep-Sea Exploration, to read more access the magazine here.