Conservation efforts began in the 1900s to save the cathedral from further damage. The correction of the Rhine river resulted in a lowering of the groundwater, the wooden substructures became rotten and the foundations started to fail and needed to be replaced. Beginning in 1909 the old foundations were underpinned. Works stopped in 1916 due to World War I. Between 1924 and 1928 the fundaments were completely reinforced by a new fundament made of concrete. A new floor, made of red marble, was constructed in this period. Architect Paul Meyer-Speer engineered a system to modify the inner walls with colorful sandstone, removing most of the paintings by Veit and restoring a look similar to the original Willigis-Bardo construction. Unfortunately this system did not withstand continuing restoration efforts, and by 1959 most of the color was gone.In World War II, Mainz was a target of Allied bombing multiple times. The cathedral was hit several times in August 1942. Most of the roofs burned, and the top level of the cloister was destroyed. The vault, however, withstood the attacks and remained intact. The damaged elements were restored as authentically as possible, a process which continued well into the 1970s. In addition, much of the glass in the cathedral was replaced.