  Water usually contains dissolved gases, and because freezing is a purifying
process these gases must be expelled for ice to form. Bubbles appear at the
freezing front and are trapped in ice, making pores. These pores come in a
range of sizes from microns to millimeters and their shapes are peculiar; never
spherical but elongated, and usually fore-aft asymmetric. We show that these
remarkable shapes result of a delicate balance between freezing, capillarity,
and mass diffusion. A non-linear ordinary differential equation suffices to
describe the bubbles, with only two non-dimensional parameters representing the
supersaturation and the freezing rate. Our experiments provide us with a large
variety of pictures of bubble shapes. We show that all of them have their
rounded tip well described by an asymptotic regime of the differential
equation, and that most of them can have their full shape quantitatively
matched by a full solution. This enables the measurement of the freezing
conditions of ice samples, and the design of freeze-cast porous materials.
Furthermore, the equation exhibits a bifurcation that explains why some bubbles
grow indefinitely and make long cylindrical ``ice worms'', well known to
glaciologists.
