  When studying tetraquark states using the method of QCD sum rules, we find
some tetraquark currents, such as $\bar u_L \gamma_\mu d_L~\bar d_L \gamma^\mu
u_L$ and $\bar u_L \gamma_\mu d_L~\bar d_R \gamma^\mu u_R$, etc. We calculate
their correlation functions using the method of operator product expansion, and
find that non-perturbative QCD effects do not contribute much to them. This
indicates the possible existence of tetraquark states composed of bare quarks
with abnormally small masses. We study the possible assignment of the $X17$ as
such a state. We argue that the Atomki experiments might observe a new type of
nuclear decay process, where sea quarks take away the extra energy of the
nucleus. A unique feature of this tetraquark assignment is that we predict two
almost degenerate states with significantly different widths.
