20,000 LEAGUES UNDER THE FERMI SEA Recently Stanford and UC Santa Barbara physicists used two alternating-current voltage sources to skew the quantum states in a tiny semiconducting quantum dot in such a way as to produce (without any net applied bias) a nonzero current through the dot. This was an experimental realization of a "Thouless pump" (named for David Thouless), which pumps electrons much as an Archimedian screw pump lifts water (Switkes et al., Science, 19 March 1999; see also the commentary in the same issue by Altshuler and Glazman). Now, Mathias Wagner (Hitachi Cambridge Laboratory, 011-44-1223-44-2911, wagner@phy.cam.ac.uk) and Fernando Sols (Universidad Autónoma de Madrid) predict that a similar principle will also apply to electrons far beneath the Fermi-sea surface. The Fermi surface or Fermi level represents (in an abstract space in which all electrons are described by their momentum vectors) the highest energy an electron may possess--at zero temperature--in the conduction band of a metal or semiconductor material. Conduction electrons, those that stray from their home atoms, are usually drawn from electrons very near the Fermi surface. Electrons with lesser energies, and occupying rungs further down on an energy-level diagram, are said to reside in the "Fermi sea" and normally do not effectively contribute to the current. Wagner and Sols suggest that with high enough ac power, the resulting pump current might actually consist mostly of electrons from far beneath the Fermi-sea surface. These subsea currents would be largely immune from temperature effects (just as submarines are less vulnerable to surface storms), a very useful property in the electronics world. (Wagner and Sols, Physical Review Letters, 22 November 1999; get text from Select Articles; see figure at Physics News Graphics.)