{"id":620233,"date":"2026-01-18T20:11:48","date_gmt":"2026-01-18T19:11:48","guid":{"rendered":"https:\/\/cronologia-universale.it\/1839-3\/"},"modified":"2026-01-18T20:11:49","modified_gmt":"2026-01-18T19:11:49","slug":"1818-2","status":"publish","type":"post","link":"https:\/\/cronologia-universale.it\/en\/1818-2\/","title":{"rendered":"1818"},"content":{"rendered":"<p>American John Cleves Symmes proposed the Hollow Earth Theory, which states that the planet is a hollow sphere with four concentric spheres inside, with enormous holes at the poles measuring 1,400 km wide. Several expeditions to the poles were planned in the following decades to find these enormous holes.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>American John Cleves Symmes proposed the Hollow Earth Theory, which states that the planet is&#8230;<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[40],"tags":[],"class_list":["post-620233","post","type-post","status-publish","format-standard","hentry","category-mt-en"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.0 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>1818 - Cronologia Universale<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/cronologia-universale.it\/en\/1818-2\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"1818 - 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