Researches actually found two such viruses, one off the coast of central Chile, and the other from a freshwater pond near Melbourne, Australia.
They propose a new genus called Pandoravirus, but unlike the mythological Pandora's Box, this underwater virus won't release any evil disease into the world.
Dr. Jean-Michel Claverie of Aix-Marseille University in France discovered the virus after a survey showed signs of them in seawater. Because the virus was so large and had such an unusual shape, it was initially misidentified as a smaller-than-normal bacteria.
They packed samples in a solution of antibiotics in order to kill an bacteria that may be present. Researchers then introduced amoebas into the bacteria-free samples, and when they died, it was clear there was something else killing them.
Large quantities of the Pandoravirus spawned, and found the giant viruses were "without morphological or genomic resemblance to any previously defined virus families," according tot he study published in Science.
Its genetic code is roughly twice the size of the record-holding Megavirus. Only 6 percent of its genes resemble anything found in other organisms.
"We believe that those new Pandoraviruses have emerged from a new ancestral cellular type that no longer exists," Claverie said.
Many are asking whether the giant virus could have come to Earth from somewhere else, such as Mars, though researchers are keeping their distance from the idea. "At this point we cannot actually disprove or disregard this type of extreme scenario," Claverie said.
It's more likely the virus could have survived by being so large, letting amoebas mistake it for bacteria -- and eat it. Infecting an amoeba cell is particularly suited to letting the virus pick up stray genes from all kinds of hosts over time, which could help explain the perplexing genetic code.