Select the main idea of the paragraph. cannons nearly always betray an old wreck at the bottom of the ocean. after a few years, when the hull decays and collapses, the cannons remain intact. their straight lines are an immediate giveaway, even when they have become completely covered with coral, for nothing natural in the sea is perfectly straight. their straight lines are an immediate giveaway. cannons nearly always betray an old wreck at the bottom of the ocean. after a few years, when the hull decays and collapses, the cannons remain intact.