https://newatlas.com/pocket-sized-vlf-antenna/59287/

The US Department of Energy's SLAC National Accelerator Laboratory has developed a compact antenna that can transmit radio signals not only over long distances through the air, but also underwater, and even through solid earth. The new four-inch (10-cm) antenna exploits the piezoelectric effect to generate Very Long Frequency (VLF) waves that normally require antennae that are miles long.