Twenty million passengers embark on cruises each year, vacationing in deluxe “floating cities” that offer everything from swimming pools to shopping malls to ice skating rinks. And the ships just keep getting bigger: The average cruise ship has doubled in size in just the last ten years.

Some engineers fear that these towering behemoths are dangerously unstable, and the recent tragedy of the Costa Concordia has raised new questions about their safety.

Now, NOVA brings together marine engineering and safety experts to reconstruct the events that led up to famous cruise disasters, including the ill-fated Concordia, the Sea Diamond, and the Oceanos.