Why is Porter Square Station, on the Red Line, so deep? The adjacent stations at Davis and Harvard aren’t. Look up, to the top of the escalator, and it feels like you’re at the base of the pyramids.

Twitterer Amber Ying (@diabola) is first out of the gate with this 2006 blurb from the Boston Globe:

The reason Porter is the deepest station in the Boston area is simple. When under construction in the early 1980s (Porter opened in 1984), a construction-related decision was made to burrow deep and build the tunnel and station in rock as opposed to soft clay, said MBTA spokesman Joe Pesaturo.

“It would have been much more expensive (and taken significantly longer) to construct the tunnel and station in the soft clay due to the earth support system that would have been necessary if a more shallow alignment had been selected,” he wrote in an e-mail.

I had no idea. NPR’s Adam Martin (@adamjmartin) chimes in:

best guess…area had many shallow tunnels used by cattlemen in late1800s/early 1900s T needed to go deeper to avoid cave-ins

Better hope those escalators keep running. The elevators will be closed for a year, starting next week — ironically, to make the station more accessible.