The Manhattan side's caisson was the next structure to be built. To ensure that it would not catch fire like its counterpart had, the Manhattan caisson was lined with fireproof plate iron. It was launched from Webb & Bell's shipyard on May 11, 1871, and maneuvered into place that September. Due to the extreme underwater air pressure inside the much deeper Manhattan caisson, many workers became sick with "the bends"—decompression sickness—during this work, despite the incorporation of airlocks (which were believed to help with decompression sickness at the time). This condition was unknown at the time and was first called "caisson disease" by the project physician, Andrew Smith. Between January 25 and May 31, 1872, Smith treated 110 cases of decompression sickness, while three workers died from the disease. When iron probes underneath the Manhattan caisson found the bedrock to be even deeper than expected, Washington Roebling halted construction due to the increased risk of decompression sickness. After the Manhattan caisson reached a depth of with an air pressure of , Washington deemed the sandy subsoil overlying the bedrock beneath to be sufficiently firm, and subsequently infilled the caisson with concrete in July 1872.