In a basement machine room at Hamburg’s Kunsthalle, sealed off behind a diagonal glass wall, rainwater collected from the museum roof filters through a foyer planter and drips through a capillary system onto a stone slab below — slowly depositing calcium into the paired shape of a stalactite and stalagmite. Bogomir Ecker started the machine on December 19, 1996, self-powered by nothing but water and gravity, and built to keep dripping at roughly 10 millimeters of growth per century until the year 2496.