Mistery in the South Atlantic

Source: GUARDACOSTA Magazine – No. 08 – Year 1966
Author: Jorge M. Montañez Santiago

• Uncharted Depths in the South Atlantic: A vast region between 42°S–48°S latitude and 30°W–50°W longitude remains largely unsurveyed, with surrounding depths reaching 4,500–5,400 metres, and containing anomalous shallow areas unusual for such a deep seabed.
• The Enigmatic Discovery by La Roche (1675): French navigator La Roche claimed to have discovered a "large and pleasant island" with a good harbour on its eastern side—an assertion that sparked a long-lasting maritime mystery.
• Centuries of Unsuccessful Searches: Between the late 17th and early 19th centuries, France and Britain sent numerous expeditions—including by Halley, La Pérouse, Colnett, and Vancouver—in an effort to locate the so-called "Great Island," all without success.
• Official Removal from Nautical Charts (1825): After a final, fruitless search by British Captain Dott in 1824–1825, the island was officially declared non-existent and was removed from international nautical maps.
• Enduring Mystery and Speculation: Despite its removal, questions remain: Was La Roche lying, mistaken, or truthful? His detailed description of a harbour on the island’s eastern side suggests he may have indeed seen something real—adding fuel to the mystery’s persistence over time.

Anyone examining a nautical chart of the South Atlantic will notice a vast area, located between parallels 42°S and 48°S and meridians 30°W and 50°W, that lacks soundings. One will also observe that the nearby depths range from 2,500 to 3,000 fathoms (approximately 4,500 to 5,400 metres).

In the western section of this oceanic region—closer to our coasts than to those of any other nation—certain events have taken place over the past three centuries which, collectively, form the subject of this article. As intriguing as the enigma itself is the connection that the mystery seems to bear with our own past. The chart reveals that the referenced region is extremely deep; yet, within it lie some peculiar shallow areas, anomalies in such an abyssal seabed. Located far from current shipping lanes, these waters have remained devoid of maritime traffic, which was redirected following the opening of the Panama Canal in 1914.

Time has once again cast a veil of solitude over the area. The mystery will remain such until a new act unfolds—perhaps the final one. Let us now follow, in chronological order, the acts and actors that take part in the slow unfolding of events on a seabed 5,000 metres deep.

The story begins three centuries ago, in 1675, when the French navigator La Roche reported having discovered a large island in the South Atlantic, describing it as "a large and pleasant island, possessing a good harbour near its eastern side."

Following La Roche’s claim, the second act unfolded over a century and a half of vigorous searching for the fabled Great Island. France and Great Britain competed eagerly, both driven by the pursuit of colonial and strategic holdings. Numerous navigators from both nations—including Halley, La Pérouse, Colnett and Vancouver—explored the region where La Roche had indicated the island’s existence: 45°S latitude, and an uncertain longitude between 37°W and 50°W.

Failure after failure turned the fame and legend of the island into a source of disrepute for its presumed discoverer. At best, the more generous assessments suggested that La Roche may have mistaken an advanced promontory of the South American coast—possibly our own Cabo Dos Bahías, situated at 45°S latitude—for an island.

Nonetheless, maritime powers continued to mark this famous Great Island on their charts. The third act unfolded with the determined search conducted by British Captain Dott in the years 1824 and 1825, which yielded negative results. It was then that the island was officially declared non-existent, and it disappeared from nautical charts around the world. Apparently, the matter ended there.

From what has occurred, three possible conclusions can be drawn:

1. La Roche lied.

2. La Roche was mistaken.

3. La Roche did not lie.

The first hypothesis, while it could be considered a strategic bluff, should only be entertained as a last resort. The second is unlikely. A distant sighting from the west might, perhaps, explain it—yet this possibility must be dismissed when considering La Roche’s description: "possessing a good harbour near its eastern side."
It is reasonable to assume that such a description could only be given by a sailor who had observed—or even explored—the harbour at close quarters.

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