a voyage and creates extra space between the top of the cargo and the top of the hold. Cargo is then free to move from one side of the ship to the other as the ship rolls. This can cause the ship to list, which, in turn, causes more cargo to shift. This kind of chain reaction can capsize a bulk carrier very quickly. The 1960 SOLAS Convention sought to control this sort of problem. These regulations required the upper ballast tanks designed in a manner to prevent shifting. They also required cargoes to be leveled, or trimmed, using excavators in the holds. The practice of trimming reduces the amount of the cargo's surface area in contact with air which has a useful side-effect: reducing the chances of spontaneous combustion in cargoes such as coal, iron, and metal shavings. Another sort of risk that can affect dry cargoes is absorption of ambient moisture. When very fine concretes and aggregates mix with water, the mud created at the bottom of the hold shifts easily and can produce a free surface effect. The only way to control these risks is by good ventilation practices and careful monitoring for the presence of water. Structural problems In 1990 alone, 20 bulk carriers sank, taking with them 94 crewmen. In 1991, 24 bulk carriers sank, killing 154. This level of loss focused attention on the safety aspects of bulk carriers, and a great deal was learned. The American Bureau of Shipping concluded that the losses were "directly traceable to failure of the cargo hold structure" and Lloyd's Register of Shipping added that the hull sides could not withstand "the combination of local corrosion, fatigue cracking and operational damage." The accident studies showed a clear pattern: Sea water enters the forward hatch, due to a large wave, a poor seal, corrosion, etc. The extra water weight in hold number one compromises the partition to hold number two, Water enters hold number two and alters the trim so much that more water enters the holds With two holds rapidly filling with water, the bow submerges and the ship quickly sinks, leaving little time for the crew to react. Previous practices had required ships to withstand the flooding of a single forward hold, but did not guard against situations where two holds would flood. The case where two after (rear) holds are flooded is no better, because the