base, basement, basis, bearing wall, bed, bedding, bedrock, belly, best, bottom, bottom side, bottommost, breech, buttocks, cut, deepest, downside, floor, flooring, fond, footing, foundation, fundament, fundamental, giveaway, ground, grounds, groundwork, half-price, hardpan, lower side, lowest, lowest layer, lowest level, marked down, nether side, nethermost, nethermost level, pavement, principle, radical, reduced, riprap, rudiment, sacrificial, seat, sill, slashed, solid ground, solid rock, stereobate, stylobate, substratum, substruction, substructure, terra firma, underbelly, underbuilding, undercarriage, undergirding, underlayer, undermost, underneath, underpinning, underside, understruction, understructure

For further exploring for "rock bottom" in Webster Dictionary Online