Abstract: A container (1) of thermoplastic material comprising a side wall (2) and a base (3), said base comprising in a cross section in a plane comprising the container's vertical axis (A), a concave inwards curved section (4) connected to the side wall (2), at least two successive stairs, each stair consisting of a straight part (5,5') forming an angle (.alpha.1, .alpha.2) with a plane (B) perpendicular to said axis (A), followed by a sharp concave outwards curve (6,6'), and a dome (8) consisting of a smooth concave outwards curve, said base (3) being symmetrical about said axis (A).

Abstract: A container (1) of thermoplastic material for containing liquids comprises an anti-bulging base (5) comprising a flat ring-shaped section (9) and a central dome-shaped section (11). The diameter of the central dome-shaped section (11) is relatively large and comprises at least 60% of the diameter of the container (1). The central dome-shaped section (11) consists of a lateral wall (15) of relatively large radius of curvature and a relative flat cap (17). The ring-shaped section (9) acts as a spring, giving away to internal pressure build-up in the container, which preferably is a household bleach bottle of polyethylene. Thus the pressure on the central dome-shaped part (11), which is designated for minimum thickness and maximum strength, is relieved.