The number one thing to never leave home without. Cavers like to have three lights per person. In urban exploration one or two per person usually works. Just be sure the group has more than one light. High-candlepower spotlights are often useful. Many adventurers also use headlamps; being as hands-free as possible is a good guideline in all equipment matters.

Pretty self-explanatory. Minimally, carry one spare set. Also, a spare for your camera/flash isn't a bad idea.

Preferably some type of sturdy work glove. It'll only take one time of grabbing a fiberglass-coated pipe or putting your hand down in a puddle of caustic goo to teach you this on your own; save the learning experiences for suckers and wear gloves.

From cutting ropes to popping latches (because you forgot your keys) to digging out that nasty metal splinter you got from leaning against an old mill press (try wearing some gloves next time), a knife is indispensable.

It can be a backpack or something smaller like a hip-pack, depending on your needs; the key is hands-free gear storage. If nothing else you want a way to carry your group's spare lights and batteries.

Consider shorts for wet work, long pants for anything else -- even in warm weather. Having as much skin as possible protected from scrapes, burns, toxic gunk, etc. is a good idea. In this vein, it's common to wear an overshirt even in hot steam tunnels. Most people stick with jeans, but for draining keep in mind that synthetics (fabrics starting with "poly-") dry much more quickly than cotton/denim. As for shoes, use your common sense. If you're getting wet, wear shoes you don't mind soaking. If you're in an abandoned building, wear something thick-soled in defense of stray nails and rusty machinery. Consider traction. Some people choose sandals for drain work; some don't feel comfortable sinking their feet directly in rank funk. Personal preference.

Depending on the type of outing, this is sometimes a necessity. Many people always carry at least one short (20-30ft) climbing rope/handline. If you have confidence in your upper body strength, you can fashion a simple climbing rope in a few minutes; the kind you remember from your old phys ed. days. Just tie a big half knot every foot or so. Alternatively, tie short loops at slightly shorter intervals, and use them as holds. A longer length of weight-bearing rope for use as a safety line is sometimes useful.

I recommend a small roll in your sack, but don't worry about it for a simple trip. It's become kind of a joke, but people in the know don't laugh -- duct tape can do everything. It'll even work for shoe patches in a pinch.

One which is rated for asbestos is what you're looking for. You can find them at Home Depot or similar hardware outlets for about $40. It's kind of cumbersome, but if you're going to spend a lot of time in steam tunnels you should have one. Unless, you're sure that there is no asbestos, which is unlikely in steam tunnels -- don't count on word of mouth or what you think asbestos "looks like" for this question. Find official documents.

Small handheld radios can be useful, especially in high-traffic areas requiring lookouts. Lapel mics and earpieces are good, as they are hands-free and inconspicuous. Remember that no radios travel very far through earth.

A lot of those pesky manhole covers can be popped with just a hammer. Those which can't will usually succumb to a small crowbar (aka prybar). A hammer is slightly more versatile, and doesn't raise as much suspicion as a crowbar. The crowbar can be useful at disused sites for crusty old hatches and doors. Be careful -- getting caught with a crowbar, especially if you also have things like rope or walkie-talkies on you (or god forbid, lockpicks) can get you a quick trip to city lodgings. "Burglarious instruments."

A water bottle is always a good idea, especially in steam tunnels. Tuck it into your ever-ready pack. A person can quickly get dehydrated when you're walking and crouching and crawling through 120 degree unventilated spaces. While you won't likely get into any real danger because of it, your endurance and enjoyment will suffer.

Many tunnel explorers prefer not to have maps beforehand, as it heightens the sense of adventure. Others like to have an idea of where to go to see things that are out of the ordinary. In drains, don't ever head downstream in an unknown drain. Maps are often inaccurate (yes, even if they come straight from the sewer department) and nothing will ruin your day like getting swept over a waterfall.

Or, a digital camera or video camera. Again, this is widely variable; some adventurers rarely leave the house without one, some just don't care that much. Keep your settings in mind and consider a beater.

If you're doing lots of photo or video work, or just want a really good look at where you're going, consider a car battery-based light rig. Labor-intensive to truck around, but sometimes useful.

Take notes and make maps. It makes the trip much more interesting, and it will help your recollection when you set up that fancy website about your exploits!

This is a must if you're draining, probably ignorable otherwise. A way to keep cameras, spare batteries, and anything else dry.