Just came across the excellent Remembering Singapore site, whose recent post on Singaporean urban legends adds a new dimension to the mission to colonise the underground:

It is said that when Singapore was building the Mass Rapid Transit (MRT) in the mid-eighties, the then-Prime Minister Lee Kuan Yew consulted the highly respectable Venerable Hong Chuan about the plan. The latter warned that the tunnelings would severely damage the excellent fengshui of the island, and the only solution was to ensure all Singaporeans carry a bagua (octagon diagram) with them.
But this was impossible among the different races and religions, so PM Lee thought of an excellent idea: to design the new $1 coin with the shape of a bagua, so that it would be carried by all Singaporeans.
This urban legend was made believable due to the coincidence of the timings: The new $1 coin was launched in September 1987, just two months before MRT began its first operation.
A further addition to the rumour was the road tax label, also in the shape of an octagon, which means every car on the roads of Singapore would be carrying a bagua too.

I’d heard that the shape of the dollar coin was intended to give everyone a bagua to carry, but it never occurred to me to wonder what sort of geomantic effect building an MRT system would have. Fascinating.