  Bound states in the continuum (BICs) represent a new paradigm in photonics
due to the full suppression of radiation losses. However, this suppression has
also hampered their direct observation. By using a double terahertz (THz)
near-field technique that allows the local excitation and detection of the THz
amplitude, we are able to map for the first time the electromagnetic field of
BICs over extended areas, unveiling the field-symmetry protection that
suppresses far-field radiation. This investigation, done for metasurfaces of
dimer rod resonators, reveals the in-plane extension and formation of BICs with
anti-symmetric phases, in agreement with coupled-dipole calculations. By
displacing the rods, we demonstrate that mirror symmetry is not a necessary
condition for BIC formation. Only $\pi$-rotation symmetry is required, making
BICs exceptionally robust to structural changes. This work makes the local
field of BICs experimentally accessible, which is crucial for the engineering
of cavities with infinite lifetimes.
