Neutrinos fascinate me. Their history is particularly interesting: they were one of the first particles hypothesized by theory and then found experimentally. They were once thought to be massless, but now we know they have a very, very tiny mass. They have antiparticle counterparts (which at first blush is confusing, considering they are electrically neutral). And they "oscillate" from one flavor to another as they travel across space! I'm pretty familiar with solar neutrino detection experiments, but last week I learned about a whole new use for these mysterious particles: discovering what's happening beneath Earth's crust.