Simplify the following text for a Grade 7 native English-speaking person.
Original: Once in 1978 when on loan in Marseille ( recovered a few days later in the city 's sewers ) , again in 1998 ( in which the museum 's curator was convicted of the theft and jailed for five years along with two accomplices ) and finally in August 2007 .
Simplified: The first time was in 1978 when on loan in Marseille , but it was found a few days later in the city 's sewers . It was stolen again in 1998 and the museum 's curator was convicted of the theft and jailed for five years along with two friends .