Comment by Kon-Peki

The most impressive aspect of the Tokyo system is not the size (largest in the world), but the willingness to spend money on it. From start to finish it took less than 15 years. Chicago’s system, just as ambitious in size, is still going despite construction starting in the mid 1970s. Although to be fair, it is “complete” and operating but with reduced holding capacity: they are waiting for a handful of stone quarries away from the city to reach their end-of-life so they can dig the tunnels to them and use them for stormwater holding.

But the problem, now that these giant things exist, is that they aren’t the panacea they were thought to be. Large storms of today’s era are dumping too much water in too short an amount of time. You just can’t build big enough.