Fell Beck, entering from the left of the image, suddenly vanishes underground over Britain's highest unbroken waterfall (110 m, or 330' ) into Main Chamber, Britain's largest (known) cavern. One wouldn't guess from this fairly unassuming pothole, but the void below could hold a cathedral; various sources cite York Minster or St. Paul's Cathedral, London, depending on metrophobic or metrocentric preferences.
The first descent was made in 1895, and salt tracing in 1900 confirmed a link through to Ingleborough Cave, but the first full, human, transit wasn't until 1983, by cave divers.
For more than fifty years, non-cavers have been able to descend to the Main Chamber by winch over the Whit and August Bank holiday weekends, thanks to the Craven and Bradford Pothole Clubs.