One of the toughest swimming competitions in the world is taking place in the Hudson River this week.

Five elite swimmers started out Monday morning on a 20-mile swim from the Bear Mountain Bridge to the Tappan Zee Bridge as part of the "8 Bridges Hudson River Swim."

Organizers say the athletes all have different personal reasons for joining the grueling race, but part of their goal is to highlight the role that Riverkeeper is doing to keep the Hudson River safe. Riverkeeper is a clean water advocacy group that works to promote the health of the river.

The swimmers will resume their competition Tuesday morning, starting from the Tappan Zee Bridge and ending at the George Washington Bridge.

The competition ends later in the week at the Verrazano Narrows Bridge.