735 ILCS 5/13-117
The lien of every mortgage or trust deed in the nature of a mortgage of record on July 1, 1915, where more than 20 years have elapsed from the time the indebtedness secured thereby is due according to its written terms, or according to any extension agreement on record on July 1, 1915, is declared to have ceased by limitation unless the holder of the indebtedness secured thereby and the then owner of the real estate within 5 years after July 1, 1915, files in the office of the recorder where the mortgage or trust deed in the nature of a mortgage is recorded, an extension agreement showing the time for which the payment of the indebtedness is extended, the time when the indebtedness will become due by the terms of the extension agreement and the amount remaining unpaid on the indebtedness, then the mortgage or trust deed in the nature of a mortgage shall continue to be a lien upon the real estate described therein for a period of 10 years from the time the indebtedness will be due as shown by the extension agreement and no longer, unless some further extension agreement is filed of record. Such extension agreements shall be acknowledged and recorded in the same manner as mortgages and trust deeds in the nature of a mortgage are required by law to be acknowledged and recorded.
735 ILCS 5/13-117