Gilbert Hemsely in Cave of the Mounds

Seeing in the Dark: Lighting a Cave

October 2022

Written by Jenny

Visitors to Cave of the Mounds bring curiosity about a multitude of topics. Seeing the wonder and appreciation in their faces as we stroll through the cave is one of the perks of working here. Of course, many have questions about the cave itself, the geology of the area or how or when the speleothems formed. What initially surprised me is how many guests wonder about the infrastructure inside of the cave. However, this does make sense because we all bring a variety of experiences and knowledge with us when we visit. One question that comes up often is about the lighting in the cave.

Guests want to know when the lights were installed? How are they powered? How did we decide where to put lights? How do we change the lightbulbs, and who does it? Let’s shed some light on this topic and answer your questions. 

Light first entered this cave on August 4, 1939, when the discovery blast opened a hole in the limestone cap over the cave. As sunlight filtered into the two gaping holes in the rock, quarry workers realized something special had been found. Within hours they began exploring inside the cave using only flashlights to guide their progress in this naturally dark cavern. Guests have the chance to recreate this experience on a Discovery Tour. Discovery Tours forgo the modern lighting system to provide visitors with the opportunity to explore using only flashlights while in the cave.

Brechler Hanneman in Cave of the Mounds
Two men explorers. One is on a rock and the other is standing in a pile of rocks. They made wisconsin history

From the discovery day until the start of the first tours in 1940, the first cave developers worked diligently to clear rubble, build walkways, and install stable lighting so visitors could have a safe and enjoyable cave experience. Lighting in the early days was minimal, little more than bare bulbs strung throughout. The first basic lighting has changed throughout the years.

A major upgrade to the lighting system took place in 1978. Gilbert Hemsley Jr. worked with Cave of the Mounds to create the lighting style in use today. Hemsley was an internationally recognized theatrical lighting specialist who worked on numerous ballets, operas, musicals, and theaters in several countries. He brought his experience and creativity to light while developing a system that would emphasize the cave’s natural beauty and unique features. Hemsley’s focus on using light for both form and function makes a visit to this cave a delightful experience for countless people each year.


Gilbert Hemsely in Cave of the Mounds

Hemsley’s placement of the lights was designed to highlight the beauty of the cave formations. The lights themselves were designed to blend in with the background. Canisters of par-can lights that you’d see in the theatre were hung from the ceiling. Faders told a story to draw your eye to one area at a time. The experience was like watching a show develop with the dynamic lighting.

Kids in Exploring caves

Several changes were made in 2012.  While the dramatic lighting installed in 1978 helped tell a story, the halogen lights themselves were somewhat problematic. The heat and light generated from the bulbs caused algae to grow. Another drawback to the halogen bulbs was the large amount of energy needed to run them, as well as their relatively short working life. Therefore, the lighting system was upgraded to the use of LED light bulbs. This reduces the scope of algae blooms, reduces overall energy needs, and minimizes the need for changing lightbulbs – an important consideration with more than 200 individual bulbs. In order to preserve the cave while accessing sensitive areas, changing the lightbulbs is a task completed by trained professionals in our crew and completed by 2020.

After 30 years, most of the parts used in the dynamic lighting were no longer made. A few of the faders and dials worked but something had to be done. A new lighting system was designed to blend in with the background. When possible, the wiring and fixtures were hidden within nooks and crannies. When placed in more open areas, a carefully designed system is used to blend the lighting structures into the surrounding backdrop. This intentional design helps visitors experience the cave with as little disruption as possible. The use of chicken wire and tinted plaster hides the wiring and shields the bare bulbs from shining directly into our eyes as we explore.

New back-lit informational signs were also added in 2020 to support visits to the cave during the time of social distancing. These signs offer information about cave formation, various fossils, and specific Cave of the Mounds formations. These signs are still in place today and are available to download in several languages.

The addition of a fixed position blacklight in 2012 added a new dimension to the tour. Guests on a standard tour have the opportunity to see a blacklight in use near the Dream River.  The UV wavelength allows visitors to see photoluminescence in action.  Now guests also have the option to rent a blacklight for their cave tour. Using a handheld blacklight enhances the caving experience by allowing visitors to see examples of photoluminescent formations throughout the cave. We also have a special blacklight tour for guests to experience a traditionally guided tour using only handheld UV lights while in the cave. This tour is truly an otherworldly experience!

When you visit Cave of the Mounds, take a moment to appreciate the careful planning and maintenance that goes into making this dark space accessible.  Nature created the colorful formations, but people bring the light so that we can see.