I grew up in a squatter area in Kowloon Tong in the 1950s. It was close to the Osborne Barracks and literally next to some RAF quaters. Much of the area was below the ground level of the surrounding area such that it looked very much like what remained of a lake which had dried up. The word "Tong" in Cantonese means 'pond'. I still remember the time when some dredging work was done there for laying some huge water pipes. The trenches were probably more than 10 feet deep. I found lots of shells of cockles in the earth that was dug up. I was curious but not quite curious enough to think about : 'oh, could this have really been a pond, or could this have been part of the sea thousands of years ago?' I would really appreciate if someone could enlighten me on this - it puzzles me even these days.
re: Kowloon Tong
That's an interesting question. The place names around that area have been moved from their original location, so the meaning of the name and the place they're attached to don't necessarily have a connection.
A 1924 map (Mapping Hong Kong, plate 4-4) shows the village named Kowloon Tong just northeast of the Boundary Street / Nathan Road junction. The village roughly where today's Kowloon Tong area is, was called Kau Lung Tsai.
It also shows quite large rivers to both areas, with low ground around them. Could they have been tidal before they got silted up, and that's when the shells were deposited there?