Strabo said that Pytheas gave an account of "what is beyond the Rhine as far as Scythia", which he, Strabo, thought was false.
In the geographers of the late Roman Republic and early Roman Empire, such as Ptolemy, Scythia stretched eastward from the mouth of the river Vistula; thus Pytheas must have described the Germanic coast of the Baltic Sea; if the statement is true, there are no other possibilities.
As to whether he explored it in person, he said that he explored the entire north in person (see under Thule above).
As the periplus was a sort of ship's log, he probably did reach the Vistula.

According to The Natural History by Pliny the Elder:

Pytheas said that the Gutones, a people of Germany, inhabited the shores of an estuary of the Ocean called Mentonomon, their territory extending a distance of six thousand stadia; that, at one day's sail from this territory, is the Isle of Abalus, upon the shores of which, amber is thrown up by the waves in spring, it being an excretion of the sea in a concrete form; as, also, that the inhabitants use this amber by way of fuel, and sell it to their neighbours, the Teutones.

The "Gutones" is a simplification of two manuscript variants, Guttonibus and Guionibus, which would be in the nominative case Guttones or Guiones, the Goths in the main opinion.
The second major manuscript variant is either Mentonomon (nominative case) or Metuonidis (genitive case).
A number of etymologies have been proposed but none very well accepted.
Amber is not actually named.
It is termed the concreti maris purgamentum, the "frozen sea's leavings" after the springtime melt.
Diodorus used ēlektron, the Greek word for amber, the object that gave its name to electricity through its ability to acquire a charge.
Pliny presented an archaic opinion, as in his time amber was a precious stone brought from the Baltic at great expense, but the Germans, he said, used it for firewood, according to Pytheas.

"Mentonomon" is unambiguously stated to be an aestuarium or "estuary" of 6000 stadia, which using the Herodotean standard of 600 feet (180 m) per stadium is 681 miles (1,096 km).
That number happens to be the distance from the mouth of the Skagerrak to the mouth of the Vistula, but no source says explicitly where the figure was taken.
Competing views, however, usually have to reinterpret "estuary" to mean something other than an estuary, as the west of the Baltic Sea is the only body of estuarial water of sufficient length in the region.

Earlier Pliny said that a large island of three days' sail from the Scythian coast called Balcia by Xenophon of Lampsacus is called Basilia by Pytheas.
It is generally understood to be the same as Abalus.
Based on the amber, the island could have been Heligoland, Zealand, the shores of Bay of Gdansk, Sambia or the Curonian Lagoon, which were historically the richest sources of amber in northern Europe.
This is the earliest use of Germania.
Strabo said that Pytheas gave an account of "what is beyond the Rhine as far as Scythia", which he, Strabo, thought was false. In the geographers of the late Roman Republic and early Roman Empire, such as Ptolemy, Scythia stretched eastward from the mouth of the river Vistula; thus Pytheas must have described the Germanic coast of the Baltic Sea; if the statement is true, there are no other possibilities. As to whether he explored it in person, he said that he explored the entire north in person (see under Thule above). As the periplus was a sort of ship's log, he probably did reach the Vistula.
According to The Natural History by Pliny the Elder:

Pytheas said that the Gutones, a people of Germany, inhabited the shores of an estuary of the Ocean called Mentonomon, their territory extending a distance of six thousand stadia; that, at one day's sail from this territory, is the Isle of Abalus, upon the shores of which, amber is thrown up by the waves in spring, it being an excretion of the sea in a concrete form; as, also, that the inhabitants use this amber by way of fuel, and sell it to their neighbours, the Teutones.
The "Gutones" is a simplification of two manuscript variants, Guttonibus and Guionibus, which would be in the nominative case Guttones or Guiones, the Goths in the main opinion. The second major manuscript variant is either Mentonomon (nominative case) or Metuonidis (genitive case). A number of etymologies have been proposed but none very well accepted. Amber is not actually named. It is termed the concreti maris purgamentum, the "frozen sea's leavings" after the springtime melt. Diodorus used ēlektron, the Greek word for amber, the object that gave its name to electricity through its ability to acquire a charge. Pliny presented an archaic opinion, as in his time amber was a precious stone brought from the Baltic at great expense, but the Germans, he said, used it for firewood, according to Pytheas.
"Mentonomon" is unambiguously stated to be an aestuarium or "estuary" of 6000 stadia, which using the Herodotean standard of 600 feet (180 m) per stadium is 681 miles (1,096 km). That number happens to be the distance from the mouth of the Skagerrak to the mouth of the Vistula, but no source says explicitly where the figure was taken. Competing views, however, usually have to reinterpret "estuary" to mean something other than an estuary, as the west of the Baltic Sea is the only body of estuarial water of sufficient length in the region.
Earlier Pliny said that a large island of three days' sail from the Scythian coast called Balcia by Xenophon of Lampsacus is called Basilia by Pytheas. It is generally understood to be the same as Abalus. Based on the amber, the island could have been Heligoland, Zealand, the shores of Bay of Gdansk, Sambia or the Curonian Lagoon, which were historically the richest sources of amber in northern Europe. This is the earliest use of Germania.
Pytheas