At Strelka Mag, a carbon sequestraion thought experiment. Emphasis mine:

During a workshop on Negative Emission Technologies (NETS), a physicist colleague did a back-of-the-envelope calculation of the volume needed to sequester enough atmospheric carbon to keep concentrations at 350ppm (after fossil fuel drawdown). Using the Great Pyramid of Giza as a scalar referent, he determined that humans would need to construct 138,462 of these structures, or roughly 3.79 great pyramids per day for one hundred years. The pyramid occurred rather spontaneously and the physicist might have chosen any number of iconic structures, man-made or natural, for purely heuristic purposes.

(This is pretty much the conceptual opposite of my $1mm Dash Button project installation proposal, which would allow museum visitors to order $1mm of Kraft Macaroni & Cheese; enough to completely fill a small gallery.)