About five meters below the surface of the Bay of Naples, marble columns still stand upright on a mosaic floor. Fish drift through doorways that Roman senators once walked through on their way to the baths. This is Baiae, the ancient resort town where Rome’s elite came to escape the summer heat, and it has been underwater for roughly two thousand years. In June 2025, a team from Italy’s national environmental research institute, ISPRA, flew a bathymetric LiDAR aircraft over the site and captured the most detailed aerial map of the submerged city ever produced. The results, published under Italy’s Marine Ecosystem Restoration (MER) program, reveal a readable urban grid of streets, villa foundations, thermal baths, and harbor infrastructure preserved on the seafloor at roughly one-square-meter resolution.
A playground for emperors, now a playground for divers
In its prime, from the late Roman Republic through the early Imperial period, Baiae was the most exclusive stretch of coastline in the Mediterranean. Julius Caesar owned property here. Emperor Nero hosted banquets in a villa perched above the harbor. The town’s natural hot springs fed a sprawling complex of bathhouses, and its sheltered bay made it an ideal anchorage for pleasure boats. The Roman poet Horace called it the most beautiful bay in the world.
What destroyed Baiae was not war or neglect but geology. The town sits inside the Campi Flegrei caldera, one of Europe’s most active volcanic systems. Over centuries, a slow process called bradyseism caused the land to sink, pulling the coastline and everything built on it beneath the waves. By the medieval period, most of the Roman waterfront was submerged.
The ruins were rediscovered in the twentieth century, and in 2002 the Italian government established the Baia Underwater Archaeological Park, one of the few places in the world where recreational divers and snorkelers can swim through an ancient city. Glass-bottom boats carry visitors who prefer to stay dry. Thousands of tourists visit each year, but until now, no single survey had captured the full extent of what lies on the seabed.
How the LiDAR survey worked
On June 5, 2025, an ISPRA aircraft equipped with a bathymetric LiDAR sensor flew over the park. The instrument fires green-wavelength laser pulses downward from the aircraft. Green light penetrates shallow seawater far better than the infrared wavelengths used in standard topographic LiDAR. Each pulse bounces off the seafloor and returns to a sensor that records the round-trip travel time, allowing depth to be calculated with centimeter-scale precision. Millions of these return points, stitched together, produce a three-dimensional surface model of the underwater terrain.
The resulting map covers the protected zone at approximately one-meter resolution, according to ISPRA’s published project summary. At that detail, the outlines of individual rooms, corridors, and courtyard walls become legible from above for the first time in a single continuous image. Previous surveys relied on piecemeal sonar passes or hand-drawn sketches compiled by divers over many years, and they left significant gaps.
The new dataset shows where walls still rise above the surrounding seabed and where roofs have collapsed, filling interior spaces with rubble and sediment. It traces the alignments of streets and picks out engineered features like ancient breakwaters, harbor basins, and terraced shorelines, which appear as straight or gently curving lines against the irregular texture of natural sand and rock. In some areas, the survey hints at structures beyond the best-known clusters of ruins, including what may be outlying villas or garden complexes at the edges of the park. Those identifications remain provisional until divers can verify them on the seafloor.
Why the timing matters
Baiae is not just an archaeological curiosity. It sits in one of the most volcanically restless zones in Europe. Since 2023, the Campi Flegrei caldera has experienced renewed seismic activity, with small earthquakes and measurable ground uplift recorded onshore by Italy’s National Institute of Geophysics and Volcanology (INGV). Whether that unrest is also affecting the submerged ruins offshore is an open question, and answering it requires a precise baseline of the seafloor’s current shape. The June 2025 LiDAR survey provides exactly that. Future flights or sonar passes can be compared against it to detect whether the ground beneath Baiae is still sinking, has stabilized, or is beginning to rise.
The survey also arrives at a moment when the park faces growing pressure from tourism and coastal development. Boat anchors, propeller wash, and shifting sediment all threaten exposed mosaics and wall tops. A high-resolution map lets park managers set anchor-exclusion zones with GPS precision, plan dive routes that steer visitors away from fragile areas, and build environmental impact assessments on hard data rather than rough estimates. ISPRA’s MER program was designed with exactly this dual purpose: the same flights that image Roman pavements also capture the contours of seagrass meadows and sediment banks that help stabilize the ruins.
What the map does not yet tell us
For all its detail, the survey leaves important questions unanswered. ISPRA has not released the raw LiDAR point cloud or processed depth grids, so independent researchers cannot yet run their own measurements or verify feature-by-feature identifications. The agency’s published account names broad categories of structures, such as streets and building layouts, but does not specify which individual mosaics or bathhouse rooms were newly resolved by the aerial pass versus already documented through decades of diving.
Depth measurements for individual features have not been published either. Knowing the exact depth of a mosaic floor or a column base would let geologists compare the current position against historical records and calculate how much the ground has moved over the past century. That comparison could directly test whether recent Campi Flegrei unrest is reshaping the underwater site.
It is also unclear how much of the high-resolution data will be made publicly available. Some national mapping projects release generalized bathymetry while restricting the finest-resolution tiles for security or licensing reasons. Whether Baiae’s one-meter grid will be published in full, shared only with heritage authorities, or held back will determine how widely the map can be used in academic research, local education, and public outreach.
A city preserved in salt water
Baiae occupies a rare position in archaeology. Most ancient cities survive as foundations and fragments, stripped of their surfaces by centuries of reuse, looting, and weather. Baiae’s submersion, while destructive in its own way, also sealed large sections of the town beneath sediment and seawater before medieval stone-robbers could reach them. Mosaics that would have been pried up for building material on land remain in place on the seafloor. Plaster walls that would have crumbled in open air survive in the low-oxygen environment beneath the waves.
The new LiDAR map does not replace the slow, painstaking work of underwater excavation. Divers will still need to descend, clear sediment, photograph surfaces, and record finds by hand. But it gives them something they have never had before: a view of the whole city at once, from above, at a resolution fine enough to plan where to look next. For a site that has been explored piecemeal for decades, that perspective is transformative.
As of June 2026, the full dataset has not yet been released to the public. When it is, it will offer not just archaeologists but geologists, marine biologists, and coastal planners a shared reference layer for one of the most extraordinary underwater landscapes in the Mediterranean. Until then, the sharpest picture of Baiae belongs to the lasers that painted it from the sky and the seafloor that has kept it, quietly, for two thousand years.
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*This article was researched with the help of AI, with human editors creating the final content.
