Breaking the buck is an extremely rare event that money market fund managers always want to avoid, but it can occur if the underlying fund investments (which are generally assumed to be completely safe) significantly drop in value. This can happen if the underlying investments suffer large losses, such as defaults or strong moves in interest rates. Several funds reached or approached this critical point (from an investor faith standpoint) during the credit crisis that occurred as a result of a drop in mortgage-related assets beginning in 2007.