Nobody knows. It was lost to history a long long time ago. However, the Ethiopian Orthodox Church claims to have it, in the Church of Our Lady Mary of Zion, in Axum, Ethiopia. The Second Book of the Maccabees says that the prophet Jerimiah hid the Ark in a cave on Mount Nebo just before the Babylonian exile -- but 2 Maccabees was written close to 500 years after that. The Lemba (an African tribe with well documented descent from Jewish refugees from Yemen) claim the ark ended up in Great Zimbabwe, where it self-destructed; a replica of that one, made about 1350 is now in a museum in Harare, Zimbabwe.