Complex sentence: By the 1990s, technologies to monitor and detect underground tests had matured to the point that tests of one kiloton or over could be detected with high probability, and in 1996 negotiations began under the auspices of the United Nations to develop a comprehensive test ban.
Split into simpler sentences: By the 1990s, technologies to monitor and detect underground tests had matured to the point that tests of one kiloton or over could be detected with high probability. In 1996, negotiations began under the auspices of the United Nations to develop a comprehensive test ban.