  In this work, we study the end-of-the-world (EOW) branes anchored to the
boundaries of BCFT${}_2$ dual to the BTZ black hole. First, we explore the
thermodynamics of the boundary system consisting of the conformal boundary and
two EOW branes. This thermodynamics is extended by the tension appearing as the
effective cosmological constant of JT black holes on the EOW branes. The
tension contribution is identified with the shadow entropy equivalent to the
boundary entropy of the BCFT${}_2$. The thermodynamics of the JT black holes
and the bulk of BCFT${}_2$ can be combined into a novel grafted thermodynamics
based on the first law. Second, we focus on the observer's view of the EOW
branes by lowering the temperature. We show that the EOW branes generate a
scale called ``reefs" inside the horizon. This scale also appears in the
grafted thermodynamics. At high temperatures, observers on the EOW branes see
their respective event horizons. The reef starts to grow relatively to the
horizon size at the temperature, $T_{grow}$. As the temperature cools down the
reef area fills the entire interior of the JT black holes at the temperature
$T_{out}$. Then, the observers recognize their horizons disappear and see the
large density of the energy flux. At this temperature, the two JT regions
become causally connected. This connected spacetime has two asymptotic $AdS_2$
boundaries with a conformal matter. Also, we comment on the grafted
thermodynamics to higher dimensions in Appendix B.
