In 1972, House Majority Leader Hale Boggs was in a twin-engine plane wih Rep Nick Begich of Alaska (along with an aide and the pilot) when the plane went down in a remote part of Alaska. It was never found.
There was an aircraft found a few years ago north of Vancouver - that disappeared during WWII. It was always assumed it crashed into the water on approach between Vancouver airport and the island. In fact, it crashed into the wilderness preserve in the mountains across the bay from Vancouver. Since the area is a source of Vancouver drinking water, it is not developed and hiking etc. in there is discouraged.
I also recall reading about a bomber found in the Greenland ice from WWII, lost while ferrying across the north Atlantic. Another fighter aircraft from WWII found in the deserts of Egypt a few years ago, it must have wandered a long way off course after one of the battles.
S. A. Andrée’s Arctic balloon expedition of 1897 - the balloon failed and the group tried to trek home, their final camp was not found until 1930. There are some amazing pictures, the film was still in their camera when found, and developed 33 years after the expedition.
I know of a small plane that was lost in Alaska. It was eventually found and was being salvaged by long-lining it out with a helicopter. The helicopter had some mechanical problems and had to dump the plane, it took another twenty years to find it again. To be fair though, they already salvaged the ID plate and the rest of the plane wasn’t all that valuable.
Here’s another notable one: an Air Force transport plane that disappeared with 44 people on it somewhere between the Yukon and Montana in 1950 and never found. That search was called off because a few weeks later the search planes were needed to find a huge bomber that crashed in BC that was only found years later.
How about this one? I entered “STENDEC” into Wikipedia’s search engine, and found this. It was a complete mystery for more than 50 years, until the wreckage and body parts started emerging from a glacial terminus.
A private plane crashed into Folsom Lake outside Sacramento many years ago and was never found. Now because of the drought, the missing pilot’s family keeps hoping the plane will show up in the receding waters, but so far no luck.