Here's episode 4 of a documentary that dives into the construction of Source BMX's epic underground skatepark:

"Over the course of two years, this story follows two brothers, Marc and Richard Moore during a huge restoration project based in the East Sussex coastal town of Hastings. Their idea was simple, to build the world's largest underground skate park underneath the promenade of Hastings seafront inside a space that dates back to the 1800's and has laid derelict for the past eighteen years."