Happy to help, /u/jellyberg. Based on the qualification of "pioneers ... going out into the wilderness and borderlands to find stuff," I'll give you a precise date: Oct. 20, 1986.

That was the date the Homestead Act of 1862 officially ended in Alaska. The Homestead Act, approved in 1862 as part of a swath of progressive legislation during the American Civil War (with conservative Southern legislators no longer serving in the U.S. Congress, quite a few long-lasting bills were passed during this time). The Homestead Act became effective on Jan. 1, 1863 and allowed American citizens to claim up to 160 acres of land for free as long as they met certain qualifications. They had to:

• be the head of a family or at least 21 years old;
• be a citizen or submit an affidavit swearing they intended to become a citizen;
• not be in rebellion against the U.S. government (remember, Civil War time);
• file a claim at a land office;
• and most importantly, inhabit and cultivate the land for at least five years. Habitation was defined as the establishment of a permanent dwelling.

The Homestead Act allowed the spread of farms and farmers across heretofore "wild" land. I use wild in quotes, because this land was already in use by American Indians, but because most Plains Natives were hunter-gatherers, the U.S. federal government viewed it as terra nullius, or land belonging to no one, because it was not being used in a way that Westerners understood.

Through the Homestead Act, Americans spread far and wide. Recent immigrants and young families alike packed up and moved westward. Railroads soon spread west, allowing the produce of these homesteads to flow east to the burgeoning cities of the East Coast.

The Homestead Act continued (with revisions) well into the 20th century, even as the amount of available federal land dwindled. On Oct. 21, 1976, the passage of the Federal Land Policy and Management Act of 1976 formally ended homesteading in the other 49 states. Owing to Alaska's wide open spaces and ample federal land, the Act allowed homesteading to continue for another 10 years. After Oct. 21, 1986, no new claims could be filed, and the Homestead Act came to an end.

Now, my answer is somewhat misleading -- homesteading still continues in Alaska to this day, albeit under a different form. The state of Alaska continues to offer land sales of state land, ostensibly for remote cabins. There is no restriction on turning that property in a year-round residence (as far as I know).

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Now for the more boring answer. Mere mapping -- not settlement, mining or resource exploration -- continues in Alaska to this day. Precise maps of every creek and watercourse simply don't exist, and the decaying permafrost means lakes are perpetually changing from year to year. Trying to map every lake and pond on the North Slope, for example, is foolish -- the terrain will change by the next summer.

But most people don't talk about precise details when they ask about when an area has been "explored." For them, I explain that the last significant ground exploration of the central Brooks Range in northern Alaska took place in the 1930s, when Robert Marshall was writing books about the need to preserve the area. Chris Allan's Arctic Citadel: A History of Exploration in the Brooks Range Region of Northern Alaska was published in 2013 and might be what you're looking for.

Strictly speaking, however, the U.S. Geological Survey didn't complete its topographical map work in Alaska until the early 1960s, and I don't think you could say that Alaska "was fully explored" until you have accurate topographical maps of the landforms along the way.