Today is the 127th birthday of Danish seismologist Inge Lehmann, as the day's Google doodle reminds us. Lehmann studied at Cambridge a University before getting an office job. Eventually she found work as a scientist, and discovered that Earth had an inner core. This was done by analysis of seismic waves caused by earthquakes. These waves have names like PKP. The strongest wave should be PKJKP, a wave passing directly through Earth's centre. The PKJKP wave has never been reliably detected. Something in Earth's core swallows the energy of earthquakes. A journey to the centre of the Earth would be an excellent place to find a small Black Hole.