Complex sentence: After two years service in the Mediterranean Sea, "Erebus" was refitted as an exploration vessel for Antarctic service and on November 21 1840, captained by James Clark Ross, she departed from Tasmania for Antarctica in company with HMS "Terror".
Split into simpler sentences: After two years service in the Mediterranean Sea, she was refitted as an exploration vessel for Antarctic service. In 1840, "Erebus", captained by James Clark Ross, departed from Tasmania for Antarctica on November 21 in company with HMS "Terror".