The line of the wind-and-wave-beaten north coast is broken by several bays (bocas). Most of them are small yet impressive inlets at the mouth of dry riverbeds. Some are spectacular sights, adorned with white sandy beaches and sand dunes, as at Boca Prins and Dos Playa. Though the beaches are incredibly beautiful, swimming here is not recommended. Amazing natural bridges were created about one million years ago, also by waves eroding the limestone coast. The longest and most visited among them fell in 2005, but one remains today in Seroe Colorado, found (with difficulty) by following some unmarked trails off the coastline.