The cruise industry markets its ocean voyages as a chance for vacationers to “return to the sea” and forget their troubles onshore.

But keeping 5,000 people afloat, fat, and happy comes with hidden environmental costs.

The 16 major cruise lines plying the waters today generated more than 1 billion gallons of sewage in 2014. Thanks to lax laws, ships can dump that sewage straight into international waters—polluting the pristine ocean near cruise destination hot spots.

This year, the industry is expecting a record 23 million passengers to hit the high seas—meaning the problem isn’t going away anytime soon.

Read more about the fight to save the world’s oceans in our “Blue Planet” series.