Mystery space

"On a green island in the Main Street traffic

is a granite arch to the dead of the  Civil War --

in the Eastlake style, all cubes  and tetrahedrons,

each end of the passage barred by an iron-lace door.

They are always locked, tho the space between is empty --

from door to door it isn't much over a yard:

break open one, you could almost touch the other.

Nobody knows what the locks were meant to guard.''

-- From "Pro Patria,'' by Constance Carrier