Found just outside Villa Opicina in the hills above Trieste, Grotta Gigante (or literally Giant Cave) may not have the most original name, but when you're widely considered to be the largest tourist cave in the entire world - including by the reputable folks at the Guinness Book - you don't really need one. First explored in 1840 and was opened to the public in 1908, however, it only became a top tourist destination after electricity was finally installed in 1957. The cave consists almost entirely of one immense chamber, which measures an mind-boggling 168m long, 76m wide and nearly 100m high. Guided tours last just under one hour, and also include admission to the newly built museum and education centre above ground.