Complex sentence: He knew Pemex had drilled exploratory wells in the region in 1951, one bored into what was described as a thick layer of andesite about 1.3 km (4,200 ft) down.
Split into simpler sentences: He knew Pemex had drilled exploratory wells in the region. In 1951, one bored into what was described as a thick layer of andesite about 1.3 km (4,200 ft) down.