  We analyze the unique capability of the existing SeaQuest experiment at
Fermilab to discover well-motivated dark sector physics by measuring displaced
electron, photon, and hadron decay signals behind a compact shield. A planned
installation of a refurbished electromagnetic calorimeter could provide
powerful new sensitivity to GeV-scale vectors, dark Higgs bosons, scalars,
axions, and inelastic and strongly interacting dark matter models. This
sensitivity is both comparable and complementary to NA62, SHiP, and FASER.
SeaQuest's ability to collect data now and over the next few years provides an
especially exciting opportunity.
