Complex sentence: The cause of the 2008 sinkhole is not yet known, but a collapse in the salt dome that Daisetta sits on is thought to be the cause and suspected to be caused by a company drilling oil out of the area.
Split into simpler sentences: The cause of the 2008 sinkhole is not yet known, but a collapse in the salt dome that Daisetta sits on is thought to be the cause. The collapse it not yet known, but suspected by a company drilling oil out of the area.