For millennia humankind has dug downward for shelter, security, and comfort. Born out of pure necessity, using nothing but the ever-present earth, humankind created architecture by subtraction. In Ethiopia, monolithic churches were hewn out of rich volcanic rock. In India, elaborate temples were carved from cliffs. And in China’s Loess Valley, underground dwellings were constructed within the porous ground.

Within much of this earthen vernacular, habitation is described by what is removed. The ground is sculpted into poché and manipulated to define interior space. Taking away, often by hand, only what is essential, humankind exists within niches of the earth’s crust. The structural limit of the material, the nature of the tools, and the intended purpose of the space define this subtractive construction.

A sparse landscape and scarcity of resources for construction often drove this innovative sculpting of the ground. After all, necessity is the mother of all invention. The economy and efficiency of this subtractive building practice is remarkable. Nothing is wasted, and nothing is superfluous. The material excavated is a resource for constructing aboveground. Depth and mass offer insulation and protection, and openings provide light and ventilation. There is an inherent sustainability and natural circularity embedded in this form of construction.

On close inspection, we see that the same instinct or logic that shaped ancient cave dwelling has been adopted beneath the contemporary city. The Yaodong in China and the troglodyte dwellings of Tunisia and Libya use the high thermal performance of the earth to buffer large fluctuations in temperature, from day to night or summer to winter. In Montreal and Toronto, citizens traverse the city below ground, avoiding the subzero temperatures above. Aptly named PATH, a 30-kilometer-long pedestrian shopping complex runs under Toronto, connecting the skyscrapers above.

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