Complex sentence: They became rock climbing partners and while based in Bristol became interested in the caves of the Mendip Hills, particularly Swildon's Hole which they believed connected to Wookey Hole Caves and proved this by putting dye into the water at Swildon and seeing it emerge at Wookey.
Split into simpler sentences: They became rock climbing partners and while based in Bristol became interested in the caves of the Mendip Hills, particularly Swildon's Hole which they believed connected to Wookey Hole Caves. They proved this by putting dye into the water at Swildon's and seeing it emerge at Wookey.