The under-floor storage area on all the large passenger jets is pressurized, but it may not be climate controlled - so it could get really cold. You should know that even pressurized planes do not keep sea-level pressure, so anything in the cargo compartment or passenger compartment will see a pressure change. A weak packet, tube or bottle carried in the plane could potentially burst or leak at altitude when the ambient pressure is lower. This is a little like packing something at sea level then driving up a mountain. Smaller planes that don't fly as high as the big jets may not be pressurized at all or may store luggage behind a pressure bulkhead in an unpressurized hold.

As a bit of useless trivia: most commercial aircraft cruise between 34,000 feet and 45,000 feet. Their pressurisations systems are designed to keep cabin pressure at level equivalent to the air pressure at 5,000 to 8,000 feet above sea level. This altitude is as high as humans can endure without the first stages of hypoxia affecting them. The first thing affected by decreased oxygen is, strangely enough, loss of night vision.

Will liquid items like insect repellent,shampoo and suntan lotion freeze in under carriage compartments on planes when flying on long flights(over8 hrs.)

There have been some truly inaccurate answers here (see the wrong answers below)! Think about it this way: Animals (dogs and horses mainly) are frequently transported beneath your feet, in the luggage holds of normal passenger planes. The luggage compartments in ALL airliners is pressurised just the same as the cabin is (around the equivalent of 8000ft altitude is most common), and if the cabin depressurises (and the oxygen masks fall), so does the luggage hold (in fact the luggage doors are one of the common reasons for cabin pressure loss). In these events, the captain makes an immediate emergency (ie rapid!) descent to 10000 ft (which the press normally refer to as 'the plane plummeted'!). Another thing to ponder, should the nay-sayers above still have difficulty agreeing: It would be complex, and probably dangerously stressful on the airframe, if two parts of the same aluminium tube were at different pressures. Lastly, the comment about freezing! No, things should not freeze in the luggage compartment (think about animal transport again), though the luggage area is often not climate controlled in the same way as the passenger cabin... It is normally cooler, but NOT freezing.