  Three-dimensional (3D) topological Dirac semimetals (TDSs) are rare but
important as a versatile platform for exploring exotic electronic properties
and topological phase transitions. A quintessential feature of TDSs is 3D Dirac
fermions associated with bulk electronic states near the Fermi level. Using
angle-resolved photoemission spectroscopy (ARPES), we have observed such bulk
Dirac cones in epitaxially-grown {\alpha}-Sn films on InSb(111), the first such
TDS system realized in an elemental form. First-principles calculations confirm
that epitaxial strain is key to the formation of the TDS phase. A phase diagram
is established that connects the 3D TDS phase through a singular point of a
zero-gap semimetal phase to a topological insulator (TI) phase. The nature of
the Dirac cone crosses over from 3D to 2D as the film thickness is reduced.
