Complex sentence: Carl Sagan also once suggested the possibility that time travelers could be here, but are disguising their existence or are not recognized as time travelers, because bringing unintentional changes to the time-space continuum might bring about undesired outcomes to those travelers.
Split into simpler sentences: Carl Sagan also once suggested the possibility that time travelers could be here, but are disguising their existence or are not recognized as time travelers. It is because bringing unintentional changes in time-space continuum can bring about undesired outcomes to those travelers.