Tomorrow, the last major milestone of the ***Big Dig|http://www.bigdig.com/*** will be complete – the southbound portion of the Zakim Bridge and Central Artery Tunnel will be open. Boston-area travelers will now be able to drive through Boston completely underground, and the construction crews will finally be able to start tearing down the “other” green monster in the city.

Meanwhile, ***this photo|http://cache.boston.com/traffic/bigdig/galleries/artery/01.jpg*** shows the #1 best reason as to WHY the central artery is going underground.