Complex sentence: Until recently, some scientists believed that the crater--a large ultra-dense footprint fall of the meteorite, which had gone underground, others--that the crater is a deep gas breakthrough, others--that it is a fragment of the Tunguska meteorite.
Split into simpler sentences: Until recently, some scientists believed that the crater, a large ultra-dense footprint fall of the meteorite, which had gone underground, let. Others believed that the crater is a deep gas breakthrough or that it is a fragment of the Tunguska meteorite.