Which hobby is referred to as spelunking?

Answer 1

Cave exploration.

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Not these days. "Spelunking" was a rather pretentious but well-meant term coined in the 1930s(?) by a couple of American cavers, and used respectably enough for a while, but has since become a slang term of derision among cavers. It was never used outside of the USA, not even in the UK which tends to copy slavishly, US slang.

The normal term throughout the English-speaking world is "caving", and its adherents are "cavers".