After mentioning the crossing (navigatio) from Berrice to Tyle, Pliny made a brief statement that:
A Tyle unius diei navigatione mare concretum a nonnullis Cronium appellatur.

"One day's sail from Thule is the frozen ocean, called by some the Cronian Sea."

The mare concretum appears to match Strabo's pepēguia thalatta and is probably the same as the topoi ("places") mentioned in Strabo's apparent description of spring drift ice, which would have stopped his voyage further north and was for him the ultimate limit of the world.
Strabo says:
Pytheas also spoke of the waters around Thule and of those places where land properly speaking no longer exists, nor sea nor air, but a mixture of these things, like a "marine lung", in which it was said that earth and water and all things are in suspension as if this something was a link between all these elements, on which one can neither walk nor sail.

The term used for "marine lung" (pleumōn thalattios) appears to refer to jellyfish of the type the ancients called sea-lung.
The latter are mentioned by Aristotle in On the Parts of Animals as being free-floating and insensate.
They are not further identifiable from what Aristotle says but some pulmones appear in Pliny as a class of insensate sea animal; specifically the halipleumon ("salt-water lung").
William Ogle, a major translator and annotator of Aristotle, attributes the name sea-lung to the lung-like expansion and contraction of the Medusae, a kind of Cnidaria, during locomotion.
The ice resembled floating circles in the water.
The modern English term for this phenomenon is pancake ice.

The association of Pytheas' observations with drift ice has long been standard in navigational literature, including Nathaniel Bowditch's American Practical Navigator, which begins Chapter 33, Ice Navigation, with Pytheas.
At its edge, sea, slush, and ice mix, surrounded by fog.
After mentioning the crossing (navigatio) from Berrice to Tyle, Pliny made a brief statement that:
A Tyle unius diei navigatione mare concretum a nonnullis Cronium appellatur.
"One day's sail from Thule is the frozen ocean, called by some the Cronian Sea."
The mare concretum appears to match Strabo's pepēguia thalatta and is probably the same as the topoi ("places") mentioned in Strabo's apparent description of spring drift ice, which would have stopped his voyage further north and was for him the ultimate limit of the world. Strabo says:
Pytheas also spoke of the waters around Thule and of those places where land properly speaking no longer exists, nor sea nor air, but a mixture of these things, like a "marine lung", in which it was said that earth and water and all things are in suspension as if this something was a link between all these elements, on which one can neither walk nor sail.
The term used for "marine lung" (pleumōn thalattios) appears to refer to jellyfish of the type the ancients called sea-lung. The latter are mentioned by Aristotle in On the Parts of Animals as being free-floating and insensate. They are not further identifiable from what Aristotle says but some pulmones appear in Pliny as a class of insensate sea animal; specifically the halipleumon ("salt-water lung"). William Ogle, a major translator and annotator of Aristotle, attributes the name sea-lung to the lung-like expansion and contraction of the Medusae, a kind of Cnidaria, during locomotion. The ice resembled floating circles in the water. The modern English term for this phenomenon is pancake ice.
The association of Pytheas' observations with drift ice has long been standard in navigational literature, including Nathaniel Bowditch's American Practical Navigator, which begins Chapter 33, Ice Navigation, with Pytheas. At its edge, sea, slush, and ice mix, surrounded by fog.
Pytheas