In 1981, I accepted a position with Aramco Services Company (ASC) in Houston TX. 'Aramco' is derived from ARabian AMerican Oil COmpany. Aramco and its sister companies are wholly-owned subsidiaries of the House of Saud. During the interview process leading up to ASC offering me a job, the recruiter had me follow him from one ASC office to another a few blocks away in downtown Houston. To do this, we took the elevator to the lobby and an escalator to the basement. From there, we walked through what was effectively an underground strip mall, corridors leading from one downtown building to another, and lined with shops and restaurants. It was a surprise because I had never seen anything like this before in my life.
The story I was told was that sometime in the recent past (pre-1981), the owner of one building opened his basement as rental space for retail establishments. The owner of the building across the street took notice of the action and did likewise. At some point, one of them proposed cutting a path beneath the city street to join the two spaces, and workers in building A could then get to the shops and offices in building B without subjecting themselves to 100°F temperatures and 98% humidity. As building owners one by one converted their basements to retail/rental space, new tunnels were added until today, the Houston tunnel system connects 95 separate buildings, is 6 miles long, and it is a piece of cake to get so lost that your only hope of rescue is to get to street level and find out where (exactly) you are.
It almost qualifies as 'a tourist attraction'. It was stunningly extensive when I left the area in 1985. It was more elaborate when I worked on contract for EXXON/Mobil in 1998, and it has certainly gotten longer since then. It was and is literally possible to walk crosstown without ever risking being rained on. In fact, when I worked for Aramco in the 80s, I often went to lunch with the local Mensa group by walking (underground) from One Allen Center at Dallas and Smith to The Shops at Houston Center, San Jacinto at McKinney. It was a longer walk than it would have been at street-level, but it was air conditioned all the way.