The A6 motorway mainly runs through the mountainous Gorski Kotar region, requiring not only large bridges and viaducts and long tunnels along the route, but also special care must be paid to protection of the environment, as the route is located in karst terrain, with numerous water supply protection zones and significant natural heritage. Risnjak National Park is located near the A6 route, and is accessed via the Delnice interchange. Due to the motorway access and its proximity to a number of seaside resorts, Risnjak is the most visited national park in Croatia. Karst terrain is especially susceptible to water pollution, so the A6 motorway is equipped with a closed water drainage system designed to channel rainwater, meltwater, and any spillages to purpose-built processing facilities. Approximately 200 karst features—caves and other types of karst features—were observed and protected during construction of the motorway. An extraordinary example of this was a cavern long by wide and tall, found during execution of the long Vrata Tunnel. The cavern was bridged by one of the tunnel tubes, which was sealed to protect the cavern and the water flowing through it.