記憶中在狂風暴雨的日子，這個城市的中心 —— 旺角、上環及跑馬地一帶，總發生水浸。雨水太猛，城市的結構既多變又複雜迂迴，雨水不能在短時間流向海港，於是湧出地面。行人路上水深及膝，馬路上水蓋過車輪。後來，有人在城市中心的地底建了像黑洞般深廣的蓄洪池，大雨時沒有即時排出大海的雨水便暫存這裡， 待晴天時存在蓄洪池中的雨中才被排出大海，解決了城市中心雨天水浸的問題。
旱季時蓄洪池閒置著。藝術家伍韶勁在位於旺角及石硤尾之間的大坑東遊樂場地下的蓄洪池，這個容量相等於四十個奧運標準泳池的地下場域，創作了《大禹之後》（After the deluge）多媒體大型裝置藝術作品，用光影在這個地下空間，說了這個現代大禹治水的故事。中國遠古時，大禹用了十三年解決了黃河水患的問題。他觀察到父親加高河堤的治水方法的錯誤，知道必須變通才能根治水患，於是想了疏道的方法，將洪水疏導到其他地方，解決問題。
八分鐘的裝置演出，觀眾先在音樂引領下來到蓄洪池的入口，然後慢慢進入在平常日子是禁區的蓄洪池。伍韶勁並沒有用藝術手法去改變這個地下世界的本貌，飄浮在光影中的白布像波浪，也是流動著的水，在這個廣闊的場域中，錯落有致地升降，從前景到後方，彷似將雨季時暫存在這裡的雨水，及被排放到大海的整個過程詩意地重現一次。伍韶勁說他的創作意念是cast light，雖然光的照耀讓事物被看見，但重點在於事物的本義，所以他並沒有在這廣深而獨特的場域中，做出滿有野心的作品，他只想讓這個地方 —— 在城市中心地下的蓄洪池 —— 被大眾知曉。
Behind each stable and livable city, there are always a group of guarding angels who are protecting it silently.
In the past, on days of heavy rainstorm, flooding always occurred in the central areas of this city, including Mong Kok, Sheung Wan and Happy Valley. Due to the torrential rain as well as the complicated structure of the city, with many twists and turns, rain water could not be discharged to the sea within a short period of time, and thus, it ended up flooding above ground. The water level went up to the knees of pedestrians, and the wheels of cars on the road were submerged under water. Later on, a water storage tank was built underground. As deep and wide as a black hole, it temporarily stores the stormwater which has yet to be discharged to the sea during heavy rain. When rain gives way to sunny weather, the stored water will then be pumped out to the sea, thereby solving the flooding problem at the city centre.
In dry months, these stormwater storage tanks are left idle. Artist Kingsley Ng has created a large-scale multimedia art installation titled After the deluge at the stormwater storage tank facility under Tai Hang Tung playground, between Mong Kok and Shek Kip Mei. It boasts a capacity equivalent to the volume of 40 Olympic-size swimming pools. In this underground space, through the use of light and shadows, Ng tells the story of Yu the Great taming the waters. In China’s distant past, a person named Yu the Great spent thirteen years to work out a solution to the flooding problem plaguing the Yellow River. He found out why his father’s method of building dikes along the riverbanks failed, and came to the understanding that he needed to think out of the box to find a solution to the flooding issue. In the end, he developed a method of redirecting the floodwater to other areas, thereby solving the problem.
The tour at the site lasts for 8 minutes, with visitors being led by music first to the entrance of the water storage tank facility, from which they slowly enter the site which is normally off limits to the public. Ng did not employ any artistic means to alter the original appearance of this underground world. White cloths floating against the light and shadows look like waves, but they also represent flowing water. Within this vast space, the mix-up of ascending and descending ground, from the landscape at the foreground to the back, seemingly portrays poetically the whole process, from the rain water being stored temporarily here during the rainy season to later being discharged to the sea. Ng said the concept behind this creation is “cast light” — even though light allows objects to be seen, the crux is the original meaning of the objects. Therefore, he has refrained from creating an ambitious piece of work within this deep, wide and unique space. He only wants to let the public know this place – the stormwater storage tank beneath the city centre.
This hardly known underground stormwater storage tank facility, along with a group of engineering staff, are always the guardian angels of this city in the rainy season.