The service said Tuesday it's allowing the fire on York Island to keep burning. It says a lightning strike likely ignited the fire during a late July storm. Visitors notified Apostle Islands National Lakeshore park staff of the fire Thursday.
Officials say it wasn't discovered sooner because flames or smoke weren't visible from the lake. Park rangers tracked the smell of smoke to an area that's less than one-tenth of an acre.
Officials say rain and humidity have kept it from spreading. They say they'll let it burn out as long as the damp conditions continue.
The island has three campsites and remains open to the public.