If you were transported to Midwest circa 1491 and asked first person you met to describe the full extent of the land, they'd almost certainly tell you that there were vast seas to the east and west. The basic conception of the continent in the Eastern Woodlands was as "Turtle Island." Whether people at any particularly time period really knew about these oceans hundreds - if not thousands - of miles away rather than accurately surmising that the land could not be infinite is impossible to say. All we can talk about is what the archaeological record tells us about trade at the time and what the historical record tells us about the activities of more recent people.

The pre-colonial trade networks of North America were extensive, well-traveled, and quite ancient. The Hopewell Period (200 BCE - 400 CE) was particularly notable for its long-distance trades. While archaeological evidence for sea-to-sea trade is pretty scant (the best we've got at the moment is a small obsidian from the California-Nevada border showing up in New Jersey around this time), east of the Rockies the trade network is more reliably established for this period. The Scioto Hopewell of modern-day Ohio in particular situated themselves as the epicenter of this trade. Their trade networks reliable brought in obsidian from Yellowstone, shells and shark teeth from the Gulf of Mexico and the Atlantic, silver from northern Ontario, and many other goods from distant lands. The distribution of these goods, especially the Yellowstone obsidian, suggests the Scioto Hopewell themselves were making many of these long journeys, rather than relying exclusively on other people to pass it along to them, one community at a time. Their artwork also depicts exotic animals not native to Ohio that they would have seen on their journeys, like grizzly bears, bighorn sheep, mountain goats, and ocelots.

Moving closer to the historical period, we do have a prominent example of a pre-colonial journey from coast to coast. I've talked about the journeys of Monacht-Ape before, so I'll be brief here. According to the account written in the 1700s from an interview with the elderly Monacht-Ape, he first traveled from Louisiana to Maine back to Louisiana. After his first journey, he set out again, this time heading northwest along the Missouri then ultimately back down the Columbia to the Pacific. He followed the Pacific coast north for a time before learning that the land would eventually end to the northwest, at which point, disheartened, he turned back.