Brighton Pavilion Estate's famous "Secret Tunnel" (which is not quite so secret since the Pavilion started promoting the heck out of it and offering tours!) links Brighton Pavilion with the Brighton Dome.
The tunnel was originally built for George IV, and depending on who you believe, was created to allow George to visit his beloved stables and horses without going outside, to allow him to visit his mistress without being seen, or to allow her to visit him without being seen. Given that the whole point of the tunnel was to give George some privacy, it's not certain which of these possible uses was considered the most important. The Pavilion Complex certainly lacked privacy if one was going between the two blocks, and it's said that a factor to the creation of the tunnel may have been George's embarrassment over his later obesity – in his later years he certainly would have been easy to spot, and may have found it unsettling to be highly visible (and vulnerable, considering how many people hated him) and not able to quickly move out of sight.
At the "Stables" end, the tunnel links to a fairly nondescript section of basement, via a curved section, presumably so that anyone passing an opened door wouldn't be able to see down the tunnel.
At the "Pavilion" end, the tunnel leads to a secret staircase that comes up behind a hidden door set into the wall of George's bedroom, which meant that he would have been able to retire to his quarters and use the tunnel whenever he felt like it, without worrying that his comings and goings (or those of his mistress) were being logged by Pavilion staff.
The tunnel itself is straight with a rounded ceiling, with a sequence of holes bored through the roof to let in daylight. These would originally have emerged into the flowerbeds and would have have been topped by unobtrusive glass domes that passers-by wouldn't necessarily notice, and that of they did notice, couldn't investigate without clambering into a flowerbed.
Nowadays, the upper windows are heavy-duty glass flush with the ground.
The top of the tunnel was semi-excavated in 2103 to reinforce its roof, so that heavy vehicles can now park behind the Dome and load and unload stage equipment without risking damage. The structural reinforcement also means that guides are now happy taking visitors on tours of the tunnel.