Well this took about 30 seconds to find:
http://dnr.wi.gov/topic/lands/VPA/
Under sec. 895.52 of the Wisconsin Statutes, landowners are generally immune from liability for injuries received by individuals recreating on their lands. This law provides liability protection to landowners for injury or death of individuals participating in outdoor recreation. Courts have consistently interpreted this statute to protect landowners in furtherance of its purpose, which is to encourage landowners to allow others to recreate on their lands.