A friend spotted these unusual pointed sand forms (pictured) on a beach in northern Lake Michigan recently. My first thought was that the shapes had resulted from rain or waves that had been breaking over the beach, but I'm still at a loss to explain how the shapes might have formed.
There are a variety of possible answers here that have two things in common: a liquid capable of binding sand together and erosion. Readers disagree about what the liquid is and whether it comes from above or below ground - Ed
• The beach surface may look bare, but there is a thriving ecosystem in muddier layers beneath. Grazing marine worms burrow through it and then excrete waste onto the surface. Gut secretions and microbial films bind each miniature "mud volcano" slightly better than the bare sand around. Gentle erosion does the rest.
Nik Kelly, Liverpool, UK
• The ...