Final answer:
The statement regarding the Shrieking Shack being accessible from Hogwarts through a secret tunnel under the Whomping Willow is true, based on the Harry Potter series.
Step-by-step explanation:
The statement that the Shrieking Shack, which is considered the most haunted building in all of Great Britain, is accessible from Hogwarts through a secret tunnel under the Whomping Willow is indeed true. This is a fact from the Harry Potter series, specifically revealed in the third book, 'Harry Potter and the Prisoner of Azkaban.' The Shack was built for Lupin to transform into a werewolf safely, and the tunnel provided a discrete passageway for him to reach it from the school. While the Shack itself is located in Hogsmeade, the only known entrance to it is through the tunnel that starts at Hogwarts, shielded by the Whomping Willow to prevent anyone from accidentally discovering it.