  The nuclear charge radius of $^{12}$Be was precisely determined using the
technique of collinear laser spectroscopy on the $2s_{1/2}\rightarrow 2p_{1/2,
3/2}$ transition in the Be$^{+}$ ion. The mean square charge radius increases
from $^{10}$Be to $^{12}$Be by $\delta <r_{\rm c}^2>^{10,12} = 0.69(5) \fm^{2}$
compared to $\delta <r_{\rm c}^2>^{10,11} = 0.49(5) \fm^{2}$ for the
one-neutron halo isotope $^{11}$Be. Calculations in the fermionic molecular
dynamics approach show a strong sensitivity of the charge radius to the
structure of $^{12}$Be. The experimental charge radius is consistent with a
breakdown of the N=8 shell closure.
