The tunnels are amazing but they also are very popular so make a reservation early (maybe before you leave the country). The tunnels themselves reveal a world that can only be seen through the tunnel tour but don't expect the experience to be without some challenges. The tunnels have a great deal of water running through lime stone as such there is a great deal of mold that would not be as noticeable for those without allergies or breathing issues. The tunnels range from wide to very narrow so know your limits. The tour guides are very knowledgeable and you will see more of the original temple then you could above ground. Where good shoes with good grip (some stairs and inclines) and bring a sweater as it can be cooler in the tunnels.
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