The MiniBooNE experiment at Fermilab reports a total excess of $638.0\ifmmode\pm\else\textpm\fi{}52.1(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}122.2(\mathrm{syst}.)$ electronlike events from a data sample corresponding to $18.75\ifmmode\times\else\texttimes\fi{}{10}^{20}$ protons-on-target in neut...