Strange History

Drake Passage Sponge Forests: The Real Eltanin Seafloor

By The Unsolved Report Editorial Team · Last reviewed 2026-06-19

In 1964 the USNS Eltanin photographed an "antenna" on the deep seafloor near Cape Horn. The truth — a carnivorous sponge — is stranger than the legend.

On August 29, 1964, a camera lowered nearly two and a half miles into the cold dark off the tip of South America captured something that looked engineered. Rising from the mud was a slim vertical stalk crowned with evenly spaced crossbars — for all the world like a television antenna, or a microwave relay, planted on the abyssal plain where no human structure had any business being. The ship that took the photograph was the USNS Eltanin, an American research vessel mapping the violent waters near the Drake Passage. The image would spend the next half century drifting between two worlds: one of fringe speculation about a machine on the seabed, and one of patient marine biology. The truth, as it turned out, is arguably stranger than the legend — and it is still being filled in today.

The Documented Facts

The Eltanin was no UFO-hunting expedition. It was a converted ice-strengthened cargo ship assigned to the National Science Foundation's U.S. Antarctic Research Program, reclassified as an oceanographic research vessel in 1962 (Wikipedia, "USNS Eltanin")). Over 52 cruises between 1962 and 1972, the ship logged some 400,000 miles and helped survey roughly 80 percent of the Southern Ocean — pioneering work in one of the least-charted regions on Earth.

The famous frame was shot west of Cape Horn at coordinates of about 59°07′S, 105°03′W, at a depth of 3,904 meters (12,808 feet) (Wikipedia, "Eltanin Antenna"). That places it in the deep waters of the southern South Pacific bordering the Drake Passage corridor — the same storm-lashed gateway between South America and Antarctica that the Eltanin spent years probing. The "antenna" stood upright, symmetrical, and unsettlingly regular.

The scientific answer arrived quietly in 1971. Oceanographers Bruce C. Heezen and Charles D. Hollister published The Face of the Deep, in which they identified the object as a deep-sea sponge then called Cladorhiza concrescens (today placed in the genus Chondrocladia). They wrote, with some understatement, that it "somewhat resembles a space-age microwave antenna" (Wikipedia, "Eltanin Antenna"). The species had actually been formally described back in 1880 by Oscar Schmidt from specimens dredged during the legendary HMS Challenger expedition (Wikipedia, "Chondrocladia concrescens"). The nineteenth-century naturalist Alexander Agassiz had even sketched its form: "a long stem ending in ramifying roots, sunk deeply into the mud," with nodes bearing four to six club-like appendages.

For decades, that identification stayed buried in specialist literature. It surfaced for the public almost by accident. In 2003, a discussion on a UFO mailing list prompted researcher Tom DeMary to track down A. F. Amos, an oceanographer who had actually sailed aboard the Eltanin in the 1960s. Amos pointed him to Heezen and Hollister's book, and DeMary posted scans of the old sponge drawings online (Wikipedia, "Eltanin Antenna"). The "alien antenna" had a name, a date, and a Victorian pedigree.

The Genuine Open Question

Here is where the wholesome mystery actually lives — not in whether the object was a machine (it wasn't), but in how little we still know about the gardens it came from. A sponge that mimics an antenna is bizarre enough. A sponge that hunts is the real shock.

Most sponges are gentle filter feeders, drawing water through their bodies to strain out bacteria. Chondrocladia and its relatives in the family Cladorhizidae abandoned that lifestyle. Science did not even recognize carnivorous sponges until 1995, when Asbestopluma hypogea was found in a Mediterranean sea cave, revealing that meat-eating is actually common across this family (Wikipedia, "Chondrocladia"). These animals snare tiny crustaceans on surfaces armored with microscopic hooked spicules, then slowly envelop and digest them.

The open question is one of inventory. We have surveyed only a sliver of the Southern Ocean floor in any detail, and every modern expedition rewrites the map. In October 2025, the Nippon Foundation–Nekton Ocean Census, working with the Schmidt Ocean Institute, confirmed 30 previously unknown deep-sea species from the region — including a new carnivorous "death-ball" sponge (Chondrocladia sp.) covered in tiny prey-trapping hooks, found near roughly 12,000 feet (Yale Environment 360). Ocean Census head of science Michelle Taylor noted that researchers had assessed "under 30 percent of the samples collected from this expedition." In other words: the Eltanin photographed one antenna-shaped resident of a neighborhood we have barely begun to count.

Theories and Interpretations

The extraterrestrial reading (speculation — not supported). Beginning in the 1960s and revived online in the 2000s, fringe writers proposed the structure was an alien or lost-civilization artifact. This interpretation has no evidentiary support. The morphology, the spicule structure, and matching specimens from the Challenger collection all point to a biological organism. We label this firmly as legend; it persists because the photograph genuinely looks artificial, not because any data favors it.

The "antenna" is a feeding adaptation (well-supported interpretation). The most compelling scientific reading is that the very geometry that fooled human eyes is a hunting tool. The harp sponge Chondrocladia lyra, described by MBARI researchers in 2012 from depths around 3,300 meters off California, arranges its branches into parallel vertical "vanes" like a harp — a layout that maximizes surface area for catching prey drifting on deep-sea currents (MBARI, "Scientists discover extraordinary new carnivorous sponge"). By analogy, the Eltanin sponge's regular crossbars may be the same trick: a living trellis tuned to the slow rain of food in a near-foodless world. Its antenna-like symmetry is, in this view, convergent engineering by evolution.

The "sponge forest" framing (emerging, partly speculative). Researchers increasingly describe dense Antarctic and sub-Antarctic sponge assemblages as structurally complex habitats — three-dimensional thickets that shelter other animals, much as kelp or coral do elsewhere. Whether the Eltanin image captured an isolated individual or the edge of such a community cannot be settled from one black-and-white frame. We flag this as a reasonable but unconfirmed extrapolation from later, better-documented surveys.

What endures is the lesson of the photograph itself. A camera went somewhere humans had never been, returned an image that looked impossible, and waited decades for the answer. The answer was not a machine. It was a hooked, patient, predatory sponge — proof that the real Drake Passage seafloor needs no invention to astonish us.

Sources & Further Reading

Wikipedia, "USNS Eltanin (T-AK-270)")
Wikipedia, "Eltanin Antenna"
Wikipedia, "Chondrocladia concrescens"
Wikipedia, "Chondrocladia"
MBARI, "Scientists discover extraordinary new carnivorous sponge"
Yale Environment 360, "'Death Ball' Sponge and Glowing Worms Among Creatures Discovered in Southern Ocean"

Sources & further reading

https://en.wikipedia.org/wiki/USNS_Eltanin_(T-AK-270)
https://en.wikipedia.org/wiki/Eltanin_Antenna
https://en.wikipedia.org/wiki/Chondrocladia_concrescens
https://en.wikipedia.org/wiki/Chondrocladia
https://www.mbari.org/news/scientists-discover-extraordinary-new-carnivorous-sponge/
https://e360.yale.edu/digest/southern-ocean-new-species
https://onlinelibrary.wiley.com/doi/abs/10.1111/ivb.12001