Why is my app terminating?

import UIKit

class ViewController: UIViewController {

    private let apiKey = "3c531cb8bc268e115000fc33ef72a6a2"

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.


        let baseURL = NSURL(string:"https://api.forecast.io/forecast/\(apiKey)/")

        let forecastURL = NSURL(string: "37.768796, -122.447933", relativeToURL: baseURL)

        let weatherData = NSData.dataWithContentsOfURL(forecastURL, options: nil, error: nil)

        println(weatherData)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}