  The phase diagram of hydrogen is one of the most important challenges in
high-pressure physics and astrophysics. Especially, the melting of dense
hydrogen is complicated by dimer dissociation, metallization and nuclear
quantum effect of protons, which together lead to a cold melting of dense
hydrogen when above 500 GPa. Nonetheless, the variation of the melting curve at
higher pressures is virtually uncharted. Here we report that using ab initio
molecular dynamics and path integral simulations based on density functional
theory, a new atomic phase is discovered, which gives an uplifting melting
curve of dense hydrogen when beyond 2 TPa, and results in a reentrant
solid-liquid transition before entering the Wigner crystalline phase of
protons. The findings greatly extend the phase diagram of dense hydrogen, and
put metallic hydrogen into the group of alkali metals, with its melting curve
closely resembling those of lithium and sodium. [With erratum. And the
structural information of C2221 phase is also provided in this version]
