The Largest "natural" area in Indiana is the Hoosier National Forest, by far; it contains over 200,000 acres. However, as with some of the state forests, it is not contiguous; there are scattered parcels all over the southern half of the state, starting below Bloomington. Also like the state forests, it has a "multiple use" designation, which means that hunting is allowed as well as hiking, horse-riding, mountain biking, etc. You have to be aware--or ask--about hunting seasons in most areas. Off-road vehicles, however, are thankfully prohibited.
The general map of the Hoosier National Forest is available at most outdoor gear stores, or from the Forest Service offices in Bedford(811 Constitution Ave. zip 47421) or Tell City(15th and Washington zip 47586); you can use this as a guide to areas that might be of interest to you, then get the appropriate topographical maps from the state DNR in Indianapolis, or from the Forest Service offices ($6). The forest has now over 240 miles of maintained trails, as reported to me in 2003 by the Forest Service. There are established campgrounds, and most of these have some trails through the nearby woods; but you may camp anywhere in the forest where temporary restrictions are not in force.
For specific locations within the Hoosier National Forest, check Alan McPherson's "Nature Walks in Southern Indiana"; you have bought this book by now, haven't you? If not, go get it. You'll find good, if dry, descriptions and usually very good directions for every parcel that is worth visiting. Ridges, hollows, waterfalls, creeks, lakes, meadows, caves and knobs; the best of Hoosier scenery is part of the forest. The Charles C. Deam Wilderness section is the only real wilderness (or as close as we'll ever get to it) in Indiana. The Wilderness encompasses 13,000 acres of contiguous Forest Service land on the south bank of the eastern half of Lake Monroe, southeast of Bloomington and with SR 446 running along its western edge. From SR 446, Tower Ridge Road proceeds into the middle of the wilderness area; pick a parking area and wander off and get lost for a few days. Except for hunting season (and even then, only especially during deer season opening day) this will be for all practical purposes uninhabitated by anyone else. A number of horsemen, hikers and backpackers visit, of course, but this is a large area, there are no hiking trails that have been maintained at all, and you're unlikely to bump into anyone. This is the one place in the state to really get WAY out of town; take advantage of it. Another noteworthy area is the Pioneer Mothers Memorial Forest, a few miles south of Paoli on SR 37 - look for a parking area on the left (northeast) side of the road as you go through a curving area that starts out going south, heads back north, and then generally south again. Here's a field report from John Birkett:
.....the most impressive stand of Black Walnut in the State. It was called "walnut cathedral", and I couldn't think of a better description. About a half dozen or more trees, growing out of a ravine, 60-70 feet to the first branch! Quite a neat story behind these trees being saved from the veneer industry. Worth a fortune!
Cautions for the state forest are an encyclopedia for all other cautions mentioned in these pages. Insects like mosquitoes and chiggers are not uncommon in the summer. The few poisonous snakes that exist in Indiana, with the exception of the dreaded massasauga rattlesnake, all are to be found here; these are the timber rattler (very rare, I think), the copperhead (more common and remarkably bad-tempered, even for a snake) and the cottonmouth in extreme southern sections. There is quite a lot of the forest in Perry County, which is so far the only county in Indiana having an infestation of the Lyme-disease bearing Lone Star tick. There are also caves, lakes and rivers throughout the forest; use normal precautions when exploring these. Also remember to take good topo maps, and a compass, if you know how to use one, or even if you don't; you can always find your way out if you keep going in one direction, this isn't the wild west. Remember there are no water supplies in the back country that can be relied on; take a filter or be prepared to boil whatever you find; and bring some with you. Also note that there has been no logging in the HNF since 1985. This is a big plus for our National Forest, and for the hikers and riders and hunters that use it. Be sure and stop by the Forest Service offices when in the area and let them know we appreciate having this resource (much as I may gripe about some sites, it's still great to have this - many states have no such areas at all).