A Crack in the Ka`bah
This is not true. It is even irrational as you indicated. The information about this crack is available only in some shi`i sources. So, please do not think about the issue and keep yourself busy with acquiring as much knowledge as you can from the Qur’an and the Sunnah of the Prophet (peace and blessings be upon him). Think about the Muslim Ummah and its problems and your role to solve or at least help in solving them.