The terrain inside the tunnel is very uneven with water on both sides... Constant water from the rocks creates divots in the dirt. There is a gate halfway back that was locked last time but even unlocked I don't think you could get past it as it has a step. The waterfalls are on uneven paths also.
From the parking area to the mouth of the tunnel would be fine. Access inside the tunnel via wheelchair would be very tricky and not advised, especially with electric as previously mentioned due to the dripping water.
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