This article covers a group of items without enough details to merit separate articles.
Environmental objects play a large part in helping or hindering players throughout the Metal Gear saga, as well as providing added depth to the scenery of whatever locale Solid Snake, Naked Snake, or Raiden finds himself in.
Gaps in walls and underneath structures that are just tall enough to accommodate a prone human. Every Metal Gear game features a variety of dead-end or open-ended (see Ventilation Shafts below) crawl spaces suitable for evading the eyes of sentries or gaining access to places unreachable at standing height.
Standard high-voltage security fences can prevent the player from gaining entry into higher-security facilities. The player must either find breaks in the fence that can be crawled through or disable the electric current by shooting out the power source.
When Snake walks in snow or wet areas, or while bleeding, he can leave behind a trail of footprints. If a guard finds these he will follow them to Snake's current position. Crawling is a slow but effective way to avoid leaving footprints in areas where sentries would be sensitive to their presence.
A laser beam or series thereof that blocks entry to particular areas, often rigged with gas, plastic explosives, or some other trap to kill an intruder instantly if triggered. Some games will allow players to disable them by destroying a power source, but the majority require simply passing through undetected. Thermal goggles or Snake's iconic cigarettes are helpful in revealing the beams as they pass, though the latter gradually sap Snake's Life because of their health risks.
An outdoor hazard that traps victims as they struggle to escape. Snake will begin sinking if he steps into a pool of quicksand, going deeper as he moves about in it. Becoming completely submerged or being knocked into the pool results in death.
These often provide good entrances to different areas as they are not patrolled by Enemy soldiers. Items are often hidden in them and they can provide good hiding places regardless of the current Alert status.
The main concern with water is that if Snake remains submerged for too long he will drown. The O2 gauge will display for any situation in which Snake's head must remain under water (i.e. a flooded vent). Metal Gear Solid 3: Snake Eater also features a variety of aquatic wildlife that can be caught and eaten for Stamina, as well as leeches that drain Stamina and must be plucked off of Naked Snake via the Survival Viewer.
Fixed machine guns used for point defense by guards - if Snake manages to avoid being cut down by their fully-automatic fire long enough, he can commandeer them to take on large numbers of pursuers at once. Be careful to watch your rear!
Powered guns that are useful for shooting down aircraft and can be hijacked in a similar manner to the Mounted Guns above. Due to their slow turning speed, however, they are much less effective for anti-personnel attacks.
As of Metal Gear Solid 2, Snake/Raiden can use hanging mode. This allows them to swing over railings and dangle from the side, evading the enemies' sight and sometimes providing an alternate route across.
Ladders (or trees, which function in a similar way) can be used by Snake/Raiden to elevate themselves to a higher point. In the case of the latter, Snake can remain at the top, staying out of the enemies sight. This can also be used as a means to cross barriers.
Naturally occurring or man-made (nigh-)bottomless pits. In most cases, anyone who steps on their cover will have a second to avoid the floor dropping out - and in extreme cases can disregard the pit altogether as their running speed far outstrips the pit's opening width.
Grates within floors that creak or clank when someone walks or runs across them, producing heightened levels of noise that can alert Enemy soldiers. This can be avoided by crawling or by rolling/flipping over them. Most famous for their presence aboard the dummy tanker USS Discovery.
A door in the floor that leads to a crawlspace underneath. This is a useful way to escape pursuit or stealthily enter a building. Depending on the current game, the player may only be able to exit upwards from trapdoors.
Locked door These can usually be unlocked by having the right key, being in an officer uniform or by knowing the radio frequency for the door.