The 'flatfish' at the bottom of the Challenger Deep
On 23 January 1960 Jacques Piccard and Don Walsh took the bathyscaphe Trieste to the bottom of the Challenger Deep, and Piccard reported seeing a flat fish like a sole at nearly 11,000 metres.
1960 CE · 11.37, 142.59 · status: disputed
The account
If true, it was the deepest fish ever seen. The claim was widely reported and repeated in textbooks.
The sceptical view
Biologists now think fish cannot live below about 8,200 m, because their body chemistry fails under that pressure. The 'fish' was most likely a sea cucumber, and Walsh later said he was unsure what they saw.
The researched reading
A sincere glimpse of a "flatfish" at the bottom of the ocean, now understood as a likely sea cucumber. Physiology rules out a fish at that depth.
| Explanation | Weight | Why |
|---|---|---|
| Something known, misidentified | 50% | Fish cannot live below about 8,200 m: their key protein-stabilising molecule (TMAO) would have to reach concentrations that reverse their osmotic balance. Jamieson and Yancey showed the "flatfish" was most likely a holothurian (sea cucumber), seen briefly through a tiny porthole in a silt cloud. |
| Undetermined | 25% | The share a generic explanation had beyond what has been shown for this case: it fits many cases like this one, but has not been demonstrated for this one. |
| Perception, memory or mind | 11% | A brief glimpse through a small porthole, with the dive’s achievement at stake, invites seeing a familiar shape; Walsh later said he was unsure. |
| A cultural story | 7% | The claim was repeated in textbooks for decades, growing into an accepted "fact" well beyond what was seen. |
| An unknown or out-of-place animal | 5% | An unknown fish surviving where physiology says fish cannot is very unlikely; no fish has been filmed below 8,336 m despite many modern dives and landers. |
| A hoax or fabrication | 1% | Piccard was sincere; no deception is suggested. |
| Genuinely anomalous | 1% | Deep-sea biology explains both the impossibility and the likely misidentification. |
Official position: No official finding; marine biologists now reject the fish sighting as physiologically impossible and favour a sea cucumber.