A ten-inch-wide, lava-encrusted hole in the ground glows red, belches red smoke which drifts along the ground, and emits screams and a very deep throbbing sound.
I got this idea from a parody of "This Old House" on Saturday Night Live in the late 80's. John Laroquette was playing Bob Vila fixing up a haunted house.
There's a big hole dug in the ground about 3 feet by 5 feet and 3 feet deep containing an ultrasonic humidifier for the smoke (much less trouble than dry ice), the amp from my car, an auto-reverse tape player, a 10" bass speaker in a 2.5 foot cube box, two smaller speakers, and a PVC tube for the hole. At the bottom of the tube a 100 watt bulb and red filter produce the light. The ultrasonic humidifier sends white fog into the tube. There's a drain in the bottom of the tube for the condensed water vapor. A cloth-covered hole in the side of the tube allows the sound to come through but keeps the mist away from the lightbulb and the audio equipment. All this is suspended from the plywood top which is simply set on the ground. It can support 300 lbs. Despite our best efforts with a short fence and scattered tree branches, several kids have stepped onto it. We usually found candy in the tube at the end of the night. After the first year, we filled the hole in the ground with eight cement blocks to make it easier to dig up. They're still down there.
The PVC is about ten inches in diameter and 18 inches long. There's a round piece of clear plastic glued inside the tube about 4 inches from the bottom with submersible (not latex) caulk. (If I were building a new one, I'd make it stronger in case someone threw something heavy into the hole by using like 1/4 inch plastic and putting several short screws through the PVC under the edge of the clear plastic. It should still be caulked to keep water from dripping down toward the lightbulb.) The 100 watt lightbulb is attached to the bottom of the tube by coat-hanger wire. Be sure to keep the bulb far enough from the plastic that it won't melt it! The red filter is several layers of a red report cover just laid onto the clear plastic. The drain is just a 1/2 inch hole drilled in the PVC just above the clear plastic. You could extend the run time of the humidifier by running a tube from the drain back to the humidifier, but run time was never a problem for us. We just let it drain into the ground.
The cloth-covered hole is about six inches square and covered with white nylon cloth to match the PVC and the fog. The subwoofer's box is screwed to the plywood top. It faces the cloth-covered hole. The smaller speakers are mounted in a loose piece of wood and sit on the humidifier facing toward the cloth-covered hole.
The PVC tube, humidifier, tape player, and amp sit in an "equipment box" screwed to and hanging from the plywood top. The box's top side and the side toward the subwoofer are missing. The tape player is inside a Ziplock bag with duct tape around the wires where they come out. The PVC mates to the plywood top with a foam rubber gasket which was cut out of a sheet of foam rubber with scissors. The humidifier's fog-out hole is attached to a 2 inch hole in the PVC through a corrugated, flexible aluminum tube sealed up with caulk. For power I just ran a plug under the plywood top and plugged into an extension cord. The 12 volts for the car amp was wired similarly to a battery charger.
The humidifier's internal humidistat is disabled by shorting it (connecting its two leads together). Otherwise it would shut itself off because the humidity in the hole quickly rises to 100%.
A round plastic sheet (cut out of a plastic drop cloth for painting) covers the plywood top and some grass. It has a hole for the foam "lava" and is covered with dirt. We drew pentagrams and "Hail Satan!" in the dirt. And we made a sign to identify the scene. You should have a low red light shining across the hole to illuminate the fog as it drifts along the ground. It looks really cool! It's also cool to have a low-hanging fake tree with scorches on it above the hole to pick up the red light from the hole.
The lava-like top is made from painted expanding spray foam insulation on top of taped-together, crumpled-up newspaper (which fills up the inside to save the expensive foam). We had some problems with the lava from kids stepping on the plywood top (not the foam itself) and flexing the foam. This caused fissures to form in the foam which made paint flake off. If I were building this now, I'd reinforce the plywood top by running three 2x2's across the bottom of the plywood. One between the subwoofer box and equipment box, and one at each end.
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