Wikipedia wrote:Paleodictyon is a pattern, usually interpreted to be a burrow, which appears in the geologic marine record beginning in the Precambrian/Early Cambrian and in modern ocean environments. Paleodictyon were first described by Giuseppe Meneghini in 1850.
Hypotheses about the origin
The question is whether these patterns are burrows of marine animals such as worms or fossilized remains of ancient organisms (sponges or algae). Observations on Paleodictyon using Euler graph theory suggest that it cannot be an excavation trace fossil, and that it must therefore be an imprint, body fossil, or be of abiotic origin.
The search for a living animal
The IMAX film Volcanoes of the Deep Sea describes the search for a living animal that produces the Paleodictyon, using the deep-water submersible DSV Alvin near volcanic vents that lie 3,500 metres (11,500 ft) underwater in the Mid-Atlantic Ridge. They found and took samples from many of the Paleodictyon nodosum honeycomb burrows, however no creatures were found in any of them. They theorized that the burrows were being used for cultivating/trapping bacteria by whichever creature created them.
And from the Okeanos Explorer:
One of the highlights of our dives in the Northwest Gulf was ROV Deep Discover finding a set of Paleodictyon holes. Similar enigmatic features have been seen in the geologic record for some 600 million years and were originally discovered in the modern oceans on a part of the central North Atlantic portion of the Mid-Atlantic Ridge. Little is known about these features, and they definitely excited our science team. We are unsure if they have ever previously been documented in the Gulf of Mexico. Image courtesy of NOAA Okeanos Explorer Program, Gulf of Mexico 2014 Expedition.
So that beasty is around since 500+ million years, has been studied for 150+ years, and we still don't know what it is?