Posting over at the Waverley Abbey thread jogged my memory of a minor occurrence in the mid-1990s when I was visiting the ruins of the temple of Mithras at Carrawburgh, on Hadrian's Wall. For those unfamiliar with the place, the ruins of the temple consist essentially of a low wall base, not standing more than a foot or so high. If you imagine a small country chapel that has been demolished to near-ground level, that would give you an idea of its dimensions. When I visited the little ruin, as I stepped across the threshold while talking to my brother, for a second or two my voice sounded oddly muffled/echoey, as if I'd just stepped into a damp, enclosed space. As my momentum carried me forward, my voice returned to normal. I returned to where I had been and tried to recreate the effect but was unable to do so. I imagine I caught a chance echo off the sides of the doorway, but it was very odd.