spelunking
Concise Oxford English Dictionary © 2008 Oxford University Press:
spelunking /spɪˈlʌŋkɪŋ/
▶noun N. Amer. the exploration of caves, especially as a hobby.
– derivatives
spelunker noun.
spelunker noun.
– origin 1940s: from obs. spelunk ‘cave’ (from L. spelunca).
'spelunking' also found in these Oxford entries:

