extreme high tide and at extreme low tide; tidal differences on the Atlantic side are very small. The lock chambers are massive concrete structures. The side walls are from thick at the bases; toward the top, where less strength is required, they taper down in steps to . The center wall between the chambers is thick and houses three galleries that run its full length. The lowest of these is a drainage tunnel; above this is a gallery for electrical cabling; and toward the top is a passageway that allows operators to gain access to the lock machinery. Filling and draining Each lock chamber requires of water to fill it from the lowered to the raised position; the same amount of water must be drained from the chamber to lower it again. Embedded in the side and centre walls are three large water culverts that are used to carry water from the lake into the chambers to raise them, and from each chamber down to the next, or to the sea, to lower them. These culverts start at a diameter of and reduce to in diameter, large enough to accommodate a train. Cross culverts branch off from these main culverts, running under the lock chambers to openings in the floors. There are fourteen cross culverts in each chamber, each with five openings; seven cross culverts from the sidewall main culverts alternate with seven from the centre wall culvert. The water is moved by gravity and is controlled by huge valves in the culverts. Each cross culvert is independently controlled. A lock chamber can be filled in as little as ten minutes. Gates The gates separating the chambers in each flight of locks must hold back a considerable weight of water, and must be both reliable and strong enough to withstand accidents, as the failure of a gate could unleash a flood of water downstream. These gates range from high, depending on position, and are thick. The tallest gates are at Miraflores, due to the large tidal range there. The heaviest leaves weigh ; the hinges themselves each weigh . Each gate has two leaves, wide, which close to a "V" shape with the point upstream. This arrangement has the effect that the force of water from the higher side pushes the ends of the gates together firmly. The gates can be opened only when, in the operating cycle, the water