Complex sentence: Over the years it was thought that the work had been obliterated during the KTH rebuilding, and was nearly forgotten while missing for more than 40 years, but was found by accident when doing repairs in the air space above the ceiling 1993.
Split into simpler sentences: Over the years it was thought that the work had been obliterated during the KTH rebuilding, and was nearly forgotten while missing for more than 40 years. It was found by accident when doing repairs in the air space above the ceiling 1993.