What real sorrow did uncle suffer?

Lindenbrock's one sorrow was that he was not able to follow the track into the center of the earth.

That same day he deposited in the archives of the town the document he had found written by Saknussemm, and he expressed his great regret that circumstances, stronger than his will, did not allow him to follow the Icelandic traveler’s track into the very center of the earth.