Over his long career, the 71-year-old Rutgers University professor has studied the impact of dumping, oil spills, and other threats to the oceans.
More recently, he spearheaded an ambitious 10-year project to catalog all the living things in the sea. On Monday, that census reported its first results, revealing thousands more species than expected and projecting that many times more have yet to be discovered.
Before all that, however, he had to find a way to get to all that volcanic activity near the Galapagos.
By 1979, he had secured use of the world's most capable deep-diving submersible craft, known as Alvin. And to help locate the vents, he recruited explorer Robert Ballard, who would later become famous for discovering the Titanic.
Grassle and Ballard set out on a real version of the fanciful voyages of Star Trek's Kirk and Spock, seeking out new life.
The world of the vents was not just alive. It was exploding with life: plumes of polychaete worms as bright as roses, rainbow-colored creatures that draped over rocks like spaghetti, mussels the size of dinner plates.
The vent communities were like nothing ever seen on Earth. No one had imagined abundant life in a sunless environment, thriving on geothermal heat and chemistry.
On Tuesday in New Brunswick, in an office lined with photos and drawings of sea life, Grassle lit up with a smile as he recalled the adventure, describing deep-sea creatures the way a child might recount cherished Christmas gifts.
"It was a privilege to be in the sub," he said, "and we wanted as many people to share it as possible." So he arranged to have a filmmaker take footage that was made into a TV documentary called Dive to the Edge of Creation.
Grassle said he still gets a chuckle out of a New Yorker cartoon - infamous among oceanographers - in which one sophisticated urban woman says to the other: "I don't know why I don't care about the bottom of the ocean, but I don't."
That's how most people saw it, said Grassle, who lives in Princeton.
A big man, tall and broad-shouldered, Grassle grew up in Cleveland, where his parents hoped he would become a doctor. He decided his career was in science, although his path to the deep sea was partly accidental, he said. A mix-up in graduate school forced him to cancel a trip to the Arctic and exchange it for a project at Woods Hole in Massachusetts.
There, he said, he went out on fishing boats, looking for life in samples he pulled up from the seafloor. "These animals had never been seen before," he said. "That just amazed me."
As he spoke, another scientist burst into the room. "I'm here to make sure Fred is not too modest," said Richard Lutz, a professor of marine ecology. In terms of the oceans, at least, "those heat vents were the most significant biological discovery of the century," Lutz said.
Lutz also pointed out that the 1980 documentary that Grassle arranged happened to win an Emmy.
For his vent explorations, Grassle won the Franklin Institute's 2009 Medal for Earth and Environmental Sciences.
It was an exotic environment that inspired all kinds of new ideas, Lutz said. Scientists began speculating that Earth's first living things might have assembled themselves in undersea heat vents.
Subsequent genetic studies have backed that up, revealing that bacteria living near the vents are extremely primitive, constituting the base of the tree of life.
Not that life there was easy. Water coming out of the vents is hot enough to melt lead, Lutz said, while the surrounding ocean is near freezing.
The environment is so hostile that it inspired a new vision among astronomers: the possibility that geothermal power may fuel communities of organisms beneath the ice-crusted ocean that covers Jupiter's moon Europa.
Grassle came to Rutgers in 1989, soon after the university decided it wanted to expand its oceanography program into a world-class marine-science school.
Lutz said he was on the search committee that chose Grassle as the first director of the university's Institute for Marine and Coastal Science, a position he held until 2008.
"We went from being unknown to being in the top 10 in the country," Lutz said.
One of Grassle's students was Cindy Van Dover, the first female pilot of the Alvin and now director of the marine laboratory at Duke University.
"He's my hero," said Van Dover, recalling that Grassle gave students free rein to pursue even long-shot ideas.
She, for example, wanted to study an unexplained organ seen on the backs of vent shrimp that she thought might be an eye - a notion that others found ridiculous, considering how little sunlight penetrated those depths.
Grassle encouraged her, and eventually she found that the structures were indeed eyes adapted to picking up the faint glow emanating from the vents themselves.
Van Dover has since made more than 100 dives in Alvin, exploring the deep-sea vents in the Pacific and others more recently discovered in the Atlantic.
She said she had never met anyone with Grassle's passion for examining the cruddy samples that come up in a box from the deep. "When he's at a table sifting the mud and getting animals out - that's when he's really in his element," she said.
It was that passion that led Grassle to realize that mud from the seafloor harbored a diversity of life to rival the world's rain forests.
His insights into ocean diversity led him to launch the Census of Marine Life in 2000. The early findings have been more than even he expected: 250,000 known organisms so far, and four times that number likely out there, ready to be discovered.
Contact staff writer Faye Flam
at 215-854-4977 or firstname.lastname@example.org.