What is the English word for something hidden for a long time?
One expression is Holy Grail. Sense 2 below, which fits your requirement, is derived from sense 1.
(in medieval legend) the cup or platter used by Christ at the Last Supper, and in which Joseph of Arimathea received Christ's blood at the Cross. Quests for it undertaken by medieval knights are described in versions of the Arthurian legends written from the early 13th century onward.
A thing which is eagerly pursued or sought after:
‘the enterprise society where profit at any cost has become the holy grail’[Oxford]
Admittedly, the Grail of legend has never actually been found, but the metaphor — of something precious being long sought — persists.
Buried perhaps?
Among other definitions includes:
- to consign to obscurity
- to cover up to hide from sight or knowledge
- Cover (someone or something) completely
Buried also would imply that one would have to put forth effort, by uncovering it to rediscover it. Just add an additional adjective if you want to emphasize how well hidden it is. eg: "deeply buried", "well buried", etc.
You might use it like so: "The once deeply buried microwave was recovered just hours ago after teams of firefighters and emergency servicemen gave their lives to bring it to the surface."
To suss out.
to inspect or investigate (something) in order to gain more knowledge, e.g.: He carefully sussed out the situation.