All the original stairs (ladders with railings) were put in by Mr. Mercer and his workmen. While working on the exit in 1888, he fell thirty feet when his rope broke. He fell on his neck and shoulders and injured his spine. He developed tuberculosis of the spine, and died twelve years later, on November 1, 1900 at the age of 46. Margarita Castruccia Mercer (Walter's wife) then became owner of the cave.

In 1901 an electric light system was placed throughout the cave, some of which can still be seen in Dante's Inferno. In 1910, a somewhat improved stairway replaced the original ladders, though it was still necessary to climb over some of the rocks through narrow passageways. Eva Mercer Stephens (Walter's daughter) and her husband Milton helped run the cave, and the cave did not change hands until 1946 upon the death of Mr. Stephens. Mrs. Mercer died at the age of ninety-five in 1961. Eva Mercer Stephens died in Murphys in 1973.

Mr. and Mrs. Sterling Carter purchased the caverns from the Mercers in 1946 changing the name to Mercer Caverns. Mrs. Carter (Vivian) was responsible for all the landscaping that so enhanced the area. She had retired from Pacific Bell (Bell Telephone) as an Operator Supervisor. Sterling Carter had retired from Bechtel Company. They relocated to Murphys because of his mining interest and as a sideline to the caverns manufactured Placer Shaker Tables for gold prospectors.

Their original plans for improvements included new metal and concrete stairs and platforms, motel, dance pavilion, swimming pool; but the only actual improvement was a new lighting system. With the aid of PG&E illumination specialists, Carter had installed an up-to-the minute lighting system that enhanced the loveliness of the caverns. There were now 48 floodlights, 150 and 300 watts, equipped with color filters.