Complex sentence: The unrelated "Truro Company" was formed by investors in 1849, which re-excavated the shaft back down to the level, when the pit flooded again, so the decision was made to drill into the ground below the bottom of the shaft.
Split into simpler sentences: The unrelated "Truro Company" was formed by investors in 1849, which re-excavated the shaft back down to the level. They faced the same issue though when the pit flooded again, so the decision was made to drill into the ground below the bottom of the shaft.