So far I have found: Hillsborough Elementary School (California, USA 1964) Westall High School (Victoria, Australia 1966) Crestview Elementary School (Florida, USA 1967) Broad Haven Primary School (Wales, UK 1977) Upton Primary School (Cheshire, UK 1977) Ariel Primary School (Ruwa, Zimbabwe 1994) I’m interested in seeing of there is a pattern in timing and location. Are they appearing approximately every decade? In specific locations (near military bases, coastal or land locked, volcanic or n...