What: Here's the note on the back of the photo:
"This is a water pumping station on which a 1,000 ton rock fell on."
When: A trawl through the newspapers bring us to 18 & 19th July, 1926, and one of the greatest floods in Hong Kong's history. The newspaper reported :
17 INCHES IN SIX HOURS
The rainfall from 10 a.m. on Sunday ((the previous day)) to 10 a.m. today was 21.43 inches ((over 54cm!)) which is a record in some respects for the Colony.
Another column described the scene shown in the photo above:
FIVE LIVES LOST
DISASTER AT PUMPING STATION
Up to the present, the most serious occurrence that can be ascertained is the collapse of No. 3 pumping station, at Pokfulam Road, which is now buried under a huge fall of earth.
An enormous boulder, weighing anywhere between 500 and 1,000 tons, had been dislodged from the top of the hillside. At about nine o'clock this morning the whole district was shaken by a rumble, accompanied by a tremendous uproar, as the boulder slid down the hillside, and, gathering impetus, charged into the rear of the engine house of the pumping station. Trees standing in the way were simply obliterated. A storehouse where there was stocked a supply of waterpipes was crushed like a matchbox, two workmen who were working in it being killed outright. The enormous mass of rock then came to rest after crashing through the rear of the main engine house.
The following enquiry  upped the estimated size of the boulder to "several thousand tons".
Where: I believe this pumping station was on what's now part of the Hong Kong University campus. The University's decription of their new Centennial Campus says it is built on the "former WSD Elliot filters site" . The 1924 map  clearly shows a pumping station at that site, just across the road from the Elliot battery.
Who: The only person visible in the photo is up in the top-left corner, giving an idea of just how massive this boulder was:
Trivia: The newspaper  noted this wasn't the greatest amount of rainfall recorded in 24-hours. That record was set in 1889, when 29.14 inches ((74cm)) fell between 6 a.m. May 29 to 6 a.m. on May 30. Does anyone know if that record still stands?
- Hongkong Telegraph, page 1, July 19, 1926.
- The enquiry was reported over several days in the Hong Kong Daily press: 1926-09-04 pg 5, 1926-09-08 pg 5, and 1926-09-30 pg 4.
- Heritage Impact Assessment for Centennial Campus Project, The University of Hong Kong.
- 1924 Map of Hong Kong.