Most terrain features found on the battlescape can, with sufficient firepower, be destroyed. UFO outer hulls can be breached with a Blaster Bomb, inner UFO hulls can sometimes be breached by a Heavy Plasma, and weaker structures can be destroyed by more conventional weaponry, such as Grenades or even sometimes Rifles.
Damage vs. terrain
The general rule is that explosions will do a fixed 50% of their rated damage to a terrain feature, and projectile/beam weapons will do between 25% and 75% of their rated damage. The 50% damage for explosions is at the center of the explosion; damage will decrease by a constant amount the further away from the center you go.
If the damage dealt exceeds the terrain's damage rating (or "armor"), the terrain will be breached or even destroyed. If the damage is less than the terrain's damage rating, the terrain will be completely unharmed. See Explosions#Tile Characteristics for a table detailing several terrain types and their damage ratings.
The outer hull of a UFO has a damage rating of 100 points: as a result of this, only Blaster Bombs (Damage = 200) can breach them. Inner hulls and doors have a damage rating of 80 points. Heavy Plasmas, with a base damage of 115, can do between 28.75 and 86.25 points of damage to terrain. 6.25 values out of a range of 57.5 possible values (6.25 / 57.5) is 0.1087, which would mean Heavy Plasma shots will breach inner UFO walls 10.87% of the time, provided the random distribution of damage values is uniform.
Tank/Laser Cannon and Hovertank/Plasma both have a base damage of 110, or a range of 27.5 to 82.5 against terrain objects. (2.5 / 55) is 0.0455, so these tanks should be able to breach inner UFO walls 4.55% of the time.
The Hovertank/Launcher's fusion ball munition has an explosive damage of 140. 50% of 140 is 70, which means a Launcher tank can never breach inner UFO walls.
Certain inner UFO walls and doors are more durable than the average inner wall. These so-called "security walls" have a damage rating of 100, like UFO outer walls, and can only be breached with the use of a Blaster Bomb. Security walls have a smooth, unsegmented appearance, with several bands running along the top. Security doors are darker than normal UFO doors and do not have a line running down their centers. The upper level of Battleships are constructed solely with security walls, although security doors are only used to enclose the central lift. See image below.
Which weapons work against other terrain types -- wood, brick, concrete walls? Ideally a table of objects vs. weapons should be added to this article.