Ie not something like a galaxy, which we never knew of once, or that we “forgot” that there was a mountain in front of our airplane, or that airplanes can be used as missiles.
“The Lost Continent”: ok. This once.
I’ll start out with those planes buried in the Burma jungle. I’ve heard it listed as 30, 60, 200. All different numbers. Either way, it’s a lot of planes to forget about.
The Amber Room.
Vinland. The Vikings lost a whole damn continent.
Some pretty huge ships have been lost at sea.
The Hanging Gardens of Babylon - If they ever existed…
Macchu Pichu or maybe Tikal.
the Lost Generation
For nearly 200 years, it was forgotten that this statue of the Buddha was made of 6 tons of solid gold: Golden Buddha (statue) - Wikipedia
Even Wiki lost that one. 
Wonderful story. Nice video with vets. And you can pay to fly a Spitfire! Who knew?
I was also thinking also there might be some ships they forgot about–sort of Twilight Zone stuff.
Isn’t the Macchu Pichu example sort of like the Coliseum, etc., at Rome, the Pyramids, Sphinx, etc.? Just saying.
I like the Amber Room. But that was stolen, and I presume broken down, although it would be nice to think some Dr. Evil is playing with his cat in it somewhere.
ETA: The Lost Chord
Botched coding fixed.
Machu Picchu was lost to the outside world from the fall of the Inca Empire in the early 1500s, until it was visited by Hiram Bingham in 1911. The locals of course knew there were ruins there, but no one outside the immediate area knew of it for almost 400 years.
Tikal was abandoned about 950 AD. While again local people were aware of the ruins, it was not visited by outsiders or made known to the outside world until 1848.
Of course, since the Coliseum, Pyramids, Sphinx, etc were in or near major cities they continued to be well known even though ruined or abandoned.
[Adama] The 13th Colony: Earth [/Adama]
There are several examples of cities that were lost.
Babylon was buried and “rediscovered” (though apparently the locals continued to use the name Babil for the mount it was buried under so it might be something like Machu Picchu).
Troy might be an even better example as it appears even the locals were ignorant of it.
Even for cities that we have always known about, there are incredible amounts of forgotten stuff buried underneath them.
And there’s a distinction between things that were lost by or forgotten about by the entire human race and things that were lost or forgotten generally but remembered or retained by a small group of people/tribe/monastery/etc. For example, the Book of Enoch (Enoch 1) was thought by most of the world to be lost until a copy was found in Ethiopia. Codex Sinaiticus was never really “lost”, it was just forgotten about by everyone outside this one monastery in Egypt where it was “found” by Tichendorf.
Well, the US Government cannot figure out where $2 billion (handed out after the New Orleans hurricane (Katrina)) went. Almost $20 billion disappeared in Iraq. The US Government probably cannot account for all kinds of stuff-mostly military equipment (most of it was misappropriated or stolen).
Moderator Note
ralph124c, this is obviously not the kind of thing called for by the OP. Let’s stick to the subject.
Colibri
General Questions Moderator
Was Akhetaten/Amarna completely lost before the modern excavations started?
This was my first answer. My second would be Angkor Wat.