The Holland Tunnel, which runs under New York's Hudson River, was opened this day in 1927. It took seven years to complete, and required 130,000 cubic yards of concrete. Not too many people know that concrete hardens under water. Today you'll learn another "hot" tip regarding concrete: It can burn your skin. The strong alkalinity of cement can cause severe chemical burns, just as any strong acid will. That's why you should always avoid prolonged exposure to wet concrete. By the time you realize you've been burned, the irreversible damage to your skin has occurred. And, it's even more damaging should it get into your eyes. Whenever you work with concrete take care: Wear rubber gloves, a long-sleeved shirt, long pants and boots. Safety eyewear is a must.