  Quantum-mechanical shell effects are expected to strongly enhance nuclear
binding on an "island of stability" of superheavy elements. The predicted
center at proton number $Z=114,120$, or $126$ and neutron number $N=184$ has
been substantiated by the recent synthesis of new elements up to $Z=118$.
However the location of the center and the extension of the island of stability
remain vague. High-precision mass spectrometry allows the direct measurement of
nuclear binding energies and thus the determination of the strength of shell
effects. Here, we present such measurements for nobelium and lawrencium
isotopes, which also pin down the deformed shell gap at $N=152$.
