The reason for his relative obscurity is one that is all too common; he is known to have written a single book, Τὰ Περὶ τοῦ Ὠκεανοῦ (On the Ocean), possibly supplemented by a second work, Περίοδος Γῆς (Around the Earth), although some scholars believe those were two alternate titles for the same account. In any case, nothing of his writing survives except for excerpts and references to him and his accomplishments in other writing, most notably in the books of such luminaries as Strabo and Pliny the Elder. Sadly, like so much of the brilliant work of antiquity, all the copies of Pytheas's book (or books) have fallen prey to the ravages of time.
What Pytheas of Massalia recorded in his writings would have been an incredible feat for anyone, and nearly beggars belief for someone in the fourth century B.C.E. He'd heard that there were these islands off the northwest coast of Gaul, inhabited by some curious folks called the Celts, and he decided to go see for himself.
How he started his travels is uncertain. At this point in history, the Carthaginians had control of the Straits of Gibraltar, and the hostilities between them and damn near everyone else in the region would have made passage a dicey affair. There's a possibility that he crossed Gaul overland and launched from the mouth of either the Garonne or Loire River into the Bay of Biscay. In any case, once he got into the Atlantic, it was off to the races.
He sailed up the west coast of England, stopping to do considerable travel on foot, up through the Irish Sea, past the Isle of Man and the Hebrides. He is the first to note that the natives called the main island Prydain -- Romanized to Britannia -- which is where we get the modern name Britain.
Then, amazingly, Pytheas kept going.
Marble sculpture of Pytheas by Auguste Ottin (1860) [Image licensed under the Creative Commons Rvalette, Pythéas, CC BY-SA 3.0]
There are credible claims that he made it as far as either Iceland or the Lofoten Islands -- which it was is uncertain. He reports being in a place "six days north of Britain," an island of "perpetual frost and snow" where, nevertheless, "the Sun never sets" -- the first known Greek to realize what happens to day length when you're above the Arctic Circle. Not content even yet, he began to head east, making his way into the Baltic Sea. There, he met a people he called the Gutones (almost certainly the Goths), who made their living trading amber, which is abundant in what is now Poland, Lithuania, and Latvia. From there he went inland, possibly traveling along the region's many navigable rivers. It's thought that he made it all the way to what was then called Scythia, along the north shore of the Black Sea.
One of the most remarkable things about Pytheas and the others who traveled with him is how well received they were. Despite the reputation for hostility that the ancient Celts, Norse, Teutons, Baltics, and Scythians all had, there's no indication Pytheas ever had any trouble -- just showing that for the most part, if you treat people with kindness and respect, they reciprocate. Even the "barbarians."
Pytheas also had a couple of striking scientific achievements -- using the changing elevation above the horizon of familiar stars as he went north, he tried to estimate the circumference of the Earth using a sextant. His measurements were off -- Eratosthenes of Cyrene did way better a hundred years later -- but still, it was a creditable attempt. Second, he is the first person known to have associated the tides with the position and phases of the Moon, a remarkable idea during a time when the celestial objects were supposed to be gods of various sorts, and there was no inkling of a law of gravitation.
Finally, Pytheas made his way back home to write his book(s), and lived the rest of his life in comfort in his villa in Massalia. But can you imagine what he must have seen and experienced? Meeting the people of the British Isles, Scandinavia, Germany, the Baltic lands, and Scythia before they'd had much chance to contact people from other cultures. Seeing the enormous expanse of the oceans and the pack ice of the Arctic from the deck of a simple sailing ship. What an adventure -- and what extraordinary curiosity, drive, and courage.
It's such a tragedy that Pytheas's original manuscript(s) have been lost; imagine what more we could learn from his voyages by reading a first-hand account. Regardless, he was an extraordinary person, someone determined to see as much of the world as he could, and who amazingly (considering the times) lived to tell his traveler's tale to the astonishment of his friends back home.