Originally posted by Zep Tepi:

Also, anything that is a "trader's distance" out in the fog counts as explored. It renders about 500m past the fog. If you see the edge of a mountain on your map, consider the whole mountain spoiled.

I take it you've had the 'pleasure' of experiencing this first hand or is the definition of "explored" hiding in some dev post that I've not seen? Inquiring minds want to know (so they decide to restart/start a new seed).