def extract_master_tarball(cls):
        """Unpack tarball of tarballs
        """
        if not os.path.exists(cls.master_tarball_file):
            cls.download_grids()

        with tarfile.open(os.path.join(ISOCHRONES, cls.master_tarball_file)) as tar:
            logging.info('Extracting {}...'.format(cls.master_tarball_file))
            tar.extractall(ISOCHRONES)
Unpack tarball of tarballs