http://metro.co.uk/2017/03/08/rabbit-hole-leads-to-700-year-old-secret-knights-templar-cave-network-6495311
https://www.belize.com/belize-blue-hole
http://www.cavediving.com/where/mexico/doom.htm
http://tpwd.texas.gov/state-parks/devils-sinkhole
http://www.dailymail.co.uk/sciencetech/article-2179622/The-Door-Hell-Giant-hole-Karakum-Desert-40-YEARS.html
http://www.roanoke.com/news/dan_casey/murder-hole-s-reputation-built-on-underground-lore/article_df175cc3-a852-5fdf-9207-571bc7d8b485.html
http://www.china.org.cn/travel/2015-10/15/content_36815159.htm
http://www.atlasobscura.com/places/woodstock-mystery-hole-skb
http://news.nationalgeographic.com/news/2010/06/100601-sinkhole-in-guatemala-2010-world-science/
http://www.independent.co.uk/news/science/mystery-of-the-siberian-holes-at-the-end-of-the-world-solved-scientists-offer-explanation-9642988.html
http://www.businessinsider.com/russian-exploding-methane-craters-global-warming-2016-3
http://earthsky.org/earth/new-explanation-for-siberias-mystery-craters
http://www.dailymail.co.uk/sciencetech/article-3630226/Huge-explosion-created-mysterious-giant-crater-Siberia-heard-100km-away-caused-clear-glow-sky.html