  The measurement of the triple Higgs coupling is a key benchmark for the LHC
and future colliders. It directly probes the Higgs potential and its
fundamental properties in connection to new physics beyond the Standard Model.
There exist two phase space regions with an enhanced sensitivity to the Higgs
self-coupling, the Higgs pair production threshold and an intermediate top pair
threshold. We show how the invariant mass distribution of the Higgs pair offers
a systematic way to extract the Higgs self-coupling, focusing on the leading
channel $pp\to hh+X\to b\bar b\ \gamma\gamma+X$. We utilize new features of the
signal events at higher energies and estimate the potential of a high-energy
upgrade of the LHC and a future hadron collider with realistic simulations. We
find that the high-energy upgrade of the LHC to 27 TeV would reach a 5$\sigma$
observation with an integrated luminosity of 2.5 ab$^{-1}$. It would have the
potential to reach 15% (30%) accuracy at the 68% (95%) confidence level to
determine the SM Higgs boson self-coupling. A future 100 TeV collider could
improve the self-coupling measurement to better than 5% (10%) at the 68% (95%)
confidence level.
