Complex sentence: One major problem which postponed the laying of the concrete foundation was the swampy ground under which flowed a subterranean lake, requiring the water to be removed by eight months of continual pumping.
Split into simpler sentences: One major problem, which postponed the laying of the concrete foundation, was the swampy ground under which flowed a subterranean lake. This required the water to be removed by eight months of continual pumping.