the latter option seems consistent with the old data of $_\Lambda^{ }$$_\Lambda^{\!\!\!10}$Be [@Danysz63], the substantially attractive $\Lambda \Lambda$ interaction has not been convincing.

In the strangeness nuclear physics, the most fundamental problem is to recognize various facets of interactions among octet baryons ($N$, $\Lambda$, $\Sigma$, $\Xi$) in a unified way. Our detailed knowledge for the $S=0$ $NN$ sector is based on the rich data of $NN$ scatterings as well as nuclear phenomena. Recent studies for $S=-1$ many-body systems such as $\Lambda$ hypernuclei have clarified interesting features of $\Lambda N$ and $\Sigma N$ interactions in spite of scarce data of the free-space scatterings. On the other hand, for baryon-baryon interactions with $S=-2$ sectors, concerned presently, experimental information has been highly limited due to the extreme difficulties of two-body scattering experiments. Therefore the observed $\Lambda \Lambda$ bond energies of double-$\Lambda$ hypernuclei should be the most reliable source for the $S=-2$ interaction, and such data play a decisive role in determining the strength of underlying $\Lambda \Lambda$ interactions.

In view of this relevance and the experimental situation, the NAGARA event is an epoch-making one which provides us with a new and firm basis for understanding the double-$\Lambda$ hypernuclei. In recent years several experiments 