We will now leave Hansen Cave and proceed to Middle cave through a man-made tunnel. The tunnels that connect the caves were built between 1937 and 1938. Visitors may feel a gust of cool air as the door to the Hansen cave tunnel is opened. Originally the tunnels did not have doors, and by connecting the caves they dramatically increased the rate of air flow within the cave system. People soon noticed that the caves were beginning to dry out as a result of this increased air flow. Water is necessary for the "growth" or development of cave formations so, in an effort to stop the artificial drying of the caves, wooden doors were put on the Hansen Entrance, the Timpanogos Entrance and the Middle Cave tunnel in 1939. Three other doors were added to the Hansen and Middle Cave tunnels in 1991 to create a true airlock between the three caves. All of these wooden doors had to be replaced by doors made of recycled plastic in 1999 because mold had begun to grow on the wooden doors.
So far the doors have been effective and have returned the caves to their current moisture level. In addition they have allowed "baby" formations to begin developing inside the tunnels.

This is cool because unlike most formations we know the exact age of these new formations. In the year 2001, the stalactites are 64 years old and still less than ¼ of an inch long. It is tempting to use this known growth rate to estimate the age of larger formations. However, because conditions change over time, growth is not constant and this will not provide an accurate measure of a speleothem's age. Hopefully however, this general rate will give you an idea about the length of a time over which these amazing formations have developed.