The Terracotta Warriors? Or the tomb itself, really.
The planet Pluto.
In my world, “thou shalt not make a small lasagne” is #11.
The cave that housed the dead sea scrolls was rather large. Its location was lost for a thousand years or so.
Wasn’t the Titanic lost for a while?
Missed Window:
From Wikipedia: A number of expeditions were mounted to find Titanic but it was not until 1 September 1985 that a Franco-American expedition succeeded.
The Explorer, possibly the first successful submarine, was lost for over 130 years. Certainly not the largest thing ever lost though.
My feet are pretty large; I sometimes lose them, only to find them in my mouth. 
Maybe I’m thinking of a different tomb, but wasn’t this the one that further in was supposed to have a scale village with mercury for the waterways? Experts supposedly have a good idea where it can be found, but the government refuses to let them excavate, is what I heard.
Well, yes. But the entire complex was lost for over 2000 years.
How about the Western/Wailing Wall in Jerusalem? It was (supposedly) buried under an accumulation of garbage until the Ottoman Sultan Sulemain located it and gave it to Jerusalem’s Jews as a place for prayer.
Huh? Unless they were cutting off a lot of their own right hands without anyone noticing, Jews hadn’t forgotten Jerusalem.
They didn’t say Jerusalem was lost. They said the wailing wall was lost…
King Tut’s tomb?
It’s bigger than my kitchen!
Losing ships to sinking is one thing. How about the Baychimo, which was spotted on and off for over 30 years after it was abandoned.
I’d be willing to bet any good Talmud student could tell you how many feet the distance is from any major landmark in Jerusalem to the Wester Wall.
There are a ton of commandments/mitzvot to preserve just such information.
Many sites ell of that story, and I’ve heard it before.
http://mosaic.lk.net/g-wall.html
Whether it’s actually true, well I don’t know.
The Amber Room wasn’t really all that big; I mean it would be difficult to lose tons of amber (or to steal it), but it was about 22x22, nothing like losing a ship.
I’m pretty sure everyone knew it was down there somewhere, it was just a question of what the Titanic’s exact position was when it went down. Is “inaccessible” the same as “lost”?
Not exactly a big item, but the body of Eva Peron disappeared for years. To make sure some devotee doesn’t try to abscond with it, the present day tomb in Argentina is a quite elaborate affair, with different subterranean levels and trapdoors.
The Argentines had originally planned some grand monument for her grave after her passing, and an enormous block of white jade was imported from Asia. I don’t know what ever happened to the jade.
~VOW