Downloading from https://www.nationalgeographic.com/animals/2019/05/watch-how-mexican-fireflies-synchronize-light-shows/ resulted a hang at its requested header.

[antdude]

Checklist

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2019.06.21
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar bug reports including closed ones
• I've read bugs section in FAQ

Verbose log

From my 64-bit Debian Jessie oldstable v8:
$ youtube-dl -v https://www.nationalgeographic.com/animals/2019/05/watch-how-mexican-fireflies-synchronize-light-shows/
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'https://www.nationalgeographic.com/animals/2019/05/watch-how-mexican-fireflies-synchronize-light-shows/']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2019.06.21
[debug] Python version 3.4.2 (CPython) - Linux-3.16.0-9-amd64-x86_64-with-debian-8.11
[debug] exe versions: ffmpeg 3.2.10-1, ffprobe 3.2.10-1, rtmpdump 2.4
[debug] Proxy map: {}
[generic] watch-how-mexican-fireflies-synchronize-light-shows: Requesting header

Description

Downloading from https://www.nationalgeographic.com/animals/2019/05/watch-how-mexican-fireflies-synchronize-light-shows/ resulted a hang at its requested header. I waited many minutes too. Also, it doesn't seem to time out at all.

However, my friend's Fedora doesn't get stuck. It purely fails:
$ youtube-dl https://www.nationalgeographic.com/animals/2019/05/watch-how-mexican-fireflies-synchronize-light-shows/
[generic] watch-how-mexican-fireflies-synchronize-light-shows: Requesting header
WARNING: Falling back on generic information extractor.
[generic] watch-how-mexican-fireflies-synchronize-light-shows: Downloading webpage
[generic] watch-how-mexican-fireflies-synchronize-light-shows: Extracting information
ERROR: Unsupported URL: https://www.nationalgeographic.com/animals/2019/05/watch-how-mexican-fireflies-synchronize-light-shows/

[mindsolve]

Well, for me the issue is presenting kind of random.
First try, it hangs like you described, second try immediately "Unsupported URL", third try (with --verbose) immediately "Unsupported URL" and now again endless hang (with --verbose).
(those tries were executed directly after another)

System: Ubuntu 18.04.2 LTS; x64

[debug] youtube-dl version 2019.06.27
[debug] Python version 2.7.15+ (CPython) - Linux-4.15.0-52-generic-x86_64-with-Ubuntu-18.04-bionic
[debug] exe versions: ffmpeg 3.4.6, ffprobe 3.4.6, rtmpdump 2.4