Complex sentence: Since then, U.S. defense authorities surveyed the island in a series of studies, collected 45,000 tonnes of soil contaminated with radioactive isotopes that were placed into a fenced area covering 24 acres on the north of the island.
Split into simpler sentences: Since then, U.S. defense authorities have surveyed the island in a series of studies. About 45,000 tonnes of soil contaminated with radioactive isotopes was collected and placed into a fenced area covering 24 acres on the north of the island.