Last November, an oddball news story caught attention around the world. A research vessel in the Coral Sea, northeast of Australia, discovered that a small island on the map didn’t actually, well, exist. It wasn’t a victim of sea level rise or a David Copperfield illusion—there just wasn’t anything there. A lot of the news coverage at the time was (quite appropriately) of the “Gee—whaddya know?” variety, but the researchers who “undiscovered” (de-scovered?) Sandy Island recently published a paper in Eos detailing answers to deeper questions—how did the island get on the map in the first place and what can we learn from its undiscovery?

The vessel, the RV Southern Surveyor, wasn’t out checking on islands to make sure they were still there. As University of Sydney PhD student Sabin Zahirovic told Ars, the research group on board was collecting measurements of the seafloor to aid in piecing together the plate tectonic history of the Coral Sea region, which is poorly understood. That included things like seafloor depth, magnetism (magnetic signatures are locked into the rock as it cools from magma at mid-ocean ridges), gravity measurements (which detect variations in rock density), and good old-fashioned rock samples.

It was with that work on their mind that they noticed something odd. “Close to New Caledonia, we noticed this blob that represented Sandy Island on our scientific charts,” Zahirovic said. “At that time we were unaware of the island's name because it did not appear in the main navigational chart, but only in our scientific charts and cached version of Google Earth.”

Lost: one island...

Curious, they wanted to go and have a look. The ship’s captain played it cautiously, charting a course that would skirt the edge of the potentially non-existent island. “It is such a remote region that it would not be a surprise if the navigational charts were incorrect—since many of the data points in that region were from Captain Cook's 18th century manual depth measurements using an immensely long rope and a lead weight. In addition, the ocean floor is an evolving environment with submarine volcanoes (called seamounts) that can pop up in a single day, meaning that even the most up-to-date maps would be of no help,” Zahirovic said.

But the instruments showed no change in depth as they passed by—the seafloor remained 1,500 meters or so below the surface. The navigational chart was correct, and their scientific charts (and Google Earth) were in error.

When they got back on land, word got out, and scientists started scouring all kinds of maps and data sets. Some thought the researchers must have made a navigational error of some kind—after all, you could see the island in the raw satellite measurements of sea surface temperature or ocean chlorophyll content. But those data sets were not, in fact, raw. A map of “known” land areas is applied to the data to eliminate measurements that don’t belong. Even digital global ocean depth charts fell prey, forcing mapped land area to be above sea level as an a priori assumption.

In Google Earth, the area showed up as a black patch with no satellite imagery. The area was supposed to be an island, but no land appeared in the satellite images, which were consequently automatically excluded.

Although the troublesome non-island found its way into public data sets, critically including the widely used World Vector Shoreline Database, it wasn’t present on many navigational charts. “It was only once we returned to shore that the Australian Hydrographic Service confirmed that they had in fact surveyed the entire region and removed the island in 1985, while the French Hydrographic Service had already removed it in 1974,” Zahirovic told Ars.

Don’t we have satellites for this?

If some groups already knew the island didn’t exist, why didn’t that become common knowledge? Zahirovic explained that hydrographic services are usually part of national navies, which may not release the data (or may release it behind a paywall). So while many of the hydrographic services got word, the information didn’t make it into public data sets. “Another reason is the cautionary approach, since nobody wants to wrongly remove an island from charts and [be responsible for] a ship running aground,” Zahirovic said. “From our discussions at the American Geophysical Meeting in December 2012, we had information that it requires Congressional approval to remove these phantom islands from charts due to the possible hazard of incorrectly removing an island.”

Those sorts of things help explain how the non-island could have persisted for so long, but where did it come from in the first place? Did some cartographer in the 1800s mischievously draw it in, knowing that a day would come when those minor pen (or quill) strokes would thoroughly trick some robots orbiting high above the Earth? Entertaining as that idea may be (imagine this person trying to explain the prank to a friend), that probably wasn’t the intent, even if it was the outcome.

The earliest major map on which Sandy Island appeared seems to have been a 1908 British admiralty chart. The map shows an island the same size and shape in Sandy Island’s location. The chart notes that the island was reported in 1876 by a whaling ship named Velocity.

There are several possibilities that could explain that report, as the Eos paper lays out. Was there a shallow island there then that has since eroded just below the waves? Probably not, as that should have been apparent in the RV Southern Surveyor’s depth measurements. Of course, it’s possible that the Velocity made a navigational error and had actually spied land somewhere else. And it’s also possible that a mistake was made somewhere along the line of hand-copied maps.

But giving the able crew of the Velocity and the cartographers who worked on those maps the benefit of the doubt, the researchers realized there was another plausible explanation. Volcanic eruptions below the ocean surface are not uncommon in the region and can result in huge “rafts” of floating pumice that drift with the currents.

In fact, the researchers note, a pumice raft from a 2001-2002 eruption near Tonga drifted within 20 kilometers of Sandy Island’s location. That wasn’t just a one-off coincidence—the currents frequently push pumice rafts from the volcanically active Tonga region westward through that general area. Perhaps the Velocity spotted one such raft and mistook it for an island, accidentally leading over a century of maps astray.

The moral from the Coral

It’s a fun story, but it’s also a lesson for scientists. Global data sets like the handy World Vector Shoreline Database have an “official” feel to them, but that doesn’t make them infallible. It can be critical, as well, to understand how each data set is created, as the processing of land areas in ocean satellite data showed.

The Sandy Island episode highlights the need for the curation of global data sets. Sabin Zahirovic told Ars, “There is very little money for establishing and maintaining scientific databases, considering it is often seen as not ‘doing’ science but just storing and managing data.” Paywalls certainly don’t make the compilation of data any easier, either.

Finally, the researchers emphasize the value of funding data-gathering voyages, whether of the traditional variety or utilizing autonomous vehicles. “It does highlight that the surface of the Moon and Mars are better explored than our oceans,” Zahirovic said, “and future efforts should be aimed at revealing the secrets of the ocean floor and conducting basic marine research. The ocean floor helps us understand how our planet's surface formed but also supports immensely unique and sensitive ecosystems that need to be catalogued and studied.”

“The media coverage of James Cameron descending into the Marianas Trench (the deepest point on Earth's surface) in a submersible in 2012 helped raise this issue, and we hope that further development of marine technologies will help us better understand our blue planet.”

Eos, 2013. DOI: 10.1002/2013EO150001 (About DOIs).