"We’ve had to move many times. Everywhere we’ve settled they destroyed."
-- Severus Snape (CC3.6)
A secret room hidden in the roots of the Whomping Willow – direct access is via a hatch at the back of the Professor Snape’s Potions classroom (CC3.5, CC3.6, CC3.7).
The last remnants of the Order of the Phoenix, Hermione Granger and Ron Weasley have been hiding here. It is one of several places that has been used since the Battle of Hogwarts to conceal the original supporters of Harry Potter and Albus Dumbledore (CC3.5, CC3.6, CC3.7).
This room only appears during the Scorpion King timeline, as described in the essay "Albus Potter and the lesson in Quantum Mechanics" by Nick Moline.
The notes for the scene describe the room's feel as:
"The underground. An atmosphere of earth, dust, and hopeless (but necessary) endeavour." (CC3.6)
From the Web
Pottermore: Harry Potter and the Cursed Child
The theatre production opened in the West End of London at the Palace Theatre on 31 July 2016: Harry Potter and the Cursed Child, Parts One and Two