The body of water appears similar to underground lakes found on Earth in Greenland and Antarctica. On Earth, microbial life persists down in the dark, frigid waters of one such lake. The ice on Mars would also shield the Martian lake from the damaging radiation that bombards the planet’s surface.

Jonathan Lunine, director of the Center for Astrophysics and Planetary Science at Cornell University, who was not involved with the research, said the finding transforms Mars from a dusty planet to yet another “ocean world” in the solar system.

“I think the more we explore Mars, the more intriguing and complex it becomes,” Dr. Lunine said.

For years, “follow the water” has been the mantra of NASA and indeed humanity’s search for life somewhere else. Without water, there is no life as we know it. In recent years, that has led the space agency to contemplate robot probes to the moons of Jupiter and Saturn, like Europa or Enceladus, where it is now known that salty oceans exist underneath thin shells of ice and where imaginative astrobiologists can envision microbes or more complex creatures.

Since humans could see through telescopes across space, Mars has been the favorite abode of imaginary life, the backyard just over the fence where the astronomer Percival Lowell imagined he could see canals and even cities webbing the orange globe. In the final evenings of this month, the planet looms like a red lantern in the East, just 35,784,871 miles from Earth — the closest it has been in 15 years.

Those early science fiction visions were dashed when the first spacecraft photos of the planet revealed a dry, cratered and lifeless-looking surface — a seemingly dead planet. In the history of Mars exploration ever since, the more we learn, the more we think it could have had a watery, perhaps life-sustaining past. The surface is scored by old gorges, canyons, beaches, ocean basins and giant volcanoes, whose eruptions could have kept things riled up on the planet. Where this water went and how, taking most of Mars’s atmosphere with it, is one of the great and ominous environmental mysteries of our time.