For me it's heights. It's weird because sometimes I can be fine with being high up, for example, I like exploring castle's and I'm normally fine walking around the walls. Sometimes however my head just goes and I have an overwhelming need to sit or lay down and make myself as big as possible. Happened once on a school trip at the top of Lincoln castle. Being high up in any kind of urban environment or built up area sets me off every time. I go light headed and dizzy, feel sick, start sweating all over and start breathing rapidly. Normally if i can hold onto something and start going downwards it eases off fairly quickly otherwise I have to basically lay down and crawl. I hate it because it stops me from seeing sights I would love to see.