Within Houston, there is a hidden city. No, we?re not talking about the tunnels, we?re talking about the behemoth oil refineries and petrochemical plants that stretch along and dominate the Houston Ship Channel, the commercial engine of the city. These industrial complexes are hidden, tucked behind a security wall of chain-link fences and armed guards. The only way to get a good glimpse is by water. Enter the M/V Sam Houston, which has been running public tours along the Houston Ship Channel since 1958. Take the 90-minute cruise and gaze at the seemingly unending sweep of twisted steel and spaghetti-network of tubes and pipes. The ride inside the air-conditioned boat is free, but reservations are required 24 hours in advance, and don?t forget to bring a photo I.D. Sadly, security precautions prohibit taking any pictures, which means you can?t see this spectacle of engineering any other way.