And every year, once a year, for the last 35 years,

20,000 people,

a third of whom don't really know how to swim at all,

wade across this massive body of water.

This is Taiwan's annual swimming carnival.

The event is a 2-mile swim with only one objective.

And if swimmers can't make it?

For an island, Taiwan has ranked consistently low

in swimming proficiency.

In 2009, only 42 percent of students said they could swim.

The fear of water dates back to the 1940s,

when the coastline was restricted

due to fear of mainland Chinese invasion.

And for that reason, everyone is required to swim in teams,

wear an official swimming cap,

and carry around a flotation device.

People get really into it and bring things like balloons,

snacks,

facekinis,

flags,

even their phones.