Ritter and Sabine, on the southwestern edge of the Sea of Tranquillity, form a contiguous pair of post-mare craters almost identical in size (30-32 km in diameter). In contrast to craters presumably of impact origin, Ritter and Sabine are characterized by low depth-to-diameter ratios, relatively smooth dark rims, and an absence of secondary craters. The fundamentally different morphological aspects of these craters compared with eumorphic craters of the same age suggest a different mode of origin. The best analogies for Ritter and Sabine are found in terrestrial caldera such as Valles, New Mexico. Ritter and Sabine exhibit a wide variety of internal features similar to those of cauldrons of subsidence, including subsidence along ring faults, postsubsidence volcanism controlled by ring fractures, and probable resurgence of magma with accompanying uplift of the caldera floor. The floor of Ritter has collapsed along multiple ring fractures and stepped crescent wedges. The central floor is uplifted along inner ring faults, creating an elevated central plateau. Domes of probable volcanic origin are located over ring fractures and on the central floor. Sabine displays a prominent arcuate ridge of coalescing domes on the periphery of the western floor, as well as a large dome that straddles the southern rim crest and a well-developed flank ridge. Such structures are unexplained by an impact hypothesis, but they are compatible with a volcanic hypothesis.