“Obscura Day is an international celebration of unusual places. It is a day of expeditions, back room tours, & exploring hidden wonders in your own hometown.” This is the third annual day run by Atlas Obscura an online aggregator of “the world’s wonders, curiosities and esoterica.”
There are strange tours happening in the USA, Europe, Austrailia and New Zealand — according to the map on their website. They have a search function for your to find events near you, but I didn’t see a full list of events.
I would recommend taking advantage of some super discounted tours with Context Travel — like the catacombs in Naples or Nazi terror in Berlin. They have things running in Paris, New York, Boston, Naples, Berlin and London for just 5 dollars/euros/pounds. These tours normally cost 40 to 100 euros, so this is a great deal. Check out their list here.
Take peak at what last year’s event was like around the world: