Physical modeling and detailed mapping of Hebes Chasma provide new insights into the crustal composition and origin of the Valles Marineris region of equatorial Mars. Hebes Chasma is a 315-km-long and 8-km-deep closed depression containing distinctive landforms that include diapirs and extensive allochthonous flows that end in pits. A central puzzle of Hebes Chasma is how and where 105 km3 of missing material disappeared. Our physical models tested the hypothesis that the chasma formed by collapse and removal of material from below. Gravity-driven collapse in the models reproduced all the chasma's main landforms as subsidence evolved from early sagging of the upper surface, to inward collapse and removal of material, to emergence of diapirs and low-gradient flows. The models and geologic evidence suggest that Hebes Mensa arched upward diapirically and raised deep stratigraphy almost level with the chasma rim. If the chasma indeed collapsed by subsurface drainage as occurred in the models, the upper 8–10 km of deposits at Hebes must have been solid to depths of ∼5 km but viscous at greater depths. The materials removed could not have consisted mainly of basalt flows; instead, they probably were a mixture of hydrated and nonhydrated salts, water ice, liquid water, and insoluble (likely basaltic) particles. The proportions of these constituents are unknown but constrained because the material drained from the subsiding chasma and apparently contributed to the outburst floods released down neighboring Echus Chasma and Kasei Valles.