I think the discussion so far has focused a bit too much on technology. You should also take into account the impact of philosophy and religion on people's views of the world they live in. That technology has advanced to some point doesn't necessitate a prevalence of critical and rational outlook. It's perfectly possible for a culture to be immersed in a tradition of superstition and mysticism, and still develope practical inventions. Customs and social structures can also set limits on how a particular technology will be used.
For example, the invention of a printing press will greatly increase the number of recorded texts in the world - but if only the clergy are literate and it is a strict taboo for any layman to learn the occult secret of reading, the invention might not change things all that much, at least over any relatively short span of time.
Technology, in and of itself, will not necessarily lead to a boost of exploration either. Exploration tends to require a motive in order to be practiced. Take for example the "discovery" of the Americas by Columbus. He couldn't attempt the voyage without first winning the support of a monarch. The reason he succeeded was not only that technology gave him the idea and made ocean-crossing possible, but because there were major political and economic motives justifying such gambles.
A densely forested region would probably remain mysterious as long as it actually remains densely forested: most people don't have a motive to venture deep into wilderness, while a few hunters or outlaws won't be enough to convince the general populace to drop their superstitions. Deforestation brought about by the growth of population, need for farmland, or the need for wood as construction material would be the most likely reason to de-mystify the place. In a fantasy setting the presense of magic or monsters might discourage such deforestation, thus helping to keep the region sparsely populated and "untamed".
Just some food for thought