Ruined Portal

A ruined portal is a naturally generated damaged nether portal, which spawns in both the Nether and the Overworld. It contains some decoration and a loot chest around it.

Generation
Ruined portals generate in all biomes in both the Nether and the Overworld. They can spawn underground, underwater, or exposed to the air. If they generate underground, they have air pockets around them. Natural terrain around ruined portals generates as netherrack. They also generate a mass of netherrack underneath them (including "stalactite"-like shapes, and this may contain blackstone deposits in the Nether). Giant ruined portals have 3 distinct designs, and normal ruined portals have 10 designs. When a ruined portal generates, it has a 5% chance to be a giant ruined portal, for about a 1.67% chance per giant portal design. This gives normal ruined portals a 95% chance, for a 9.5% chance per normal ruined portal design.

Ruined portals that generate underground do so at Y-level from 15 to, where is the highest block at the point of generation and  is the height of the ruined portal structure. This means the terrain is always higher than or level with the top of the structure.

Ruined portals that generate in mountains generate at y-level from 70 to.

Ruined portals that generate partially buried do so at y-level, plus a random integer from 2 to 8. This means that the structure has 2 to 8 layers raised above the surface.

In the Nether, ruined portals with air pockets generate from Y-level 32 to 100. Ruined portals without air pockets have a 50% chance to spawn from Y-level 27 to 29, and a 50% chance to spawn from Y-level 29 to 100.

Many blocks in ruined portals are replaced upon generation. A ruined portal is in a snowy biome if the temperature is less than 0.15.

Structure
Ruined portals generate incomplete portal frames composed of both obsidian and crying obsidian. Note that nether portals cannot be activated with crying obsidian in the frame. Some frames generate flat on the ground, as if they toppled over. Others are free-standing separately, as if still being assembled /attached.

All ruined portals generate with a chest (quite-possibly replaced by magma block or lava in a giant - or regular - portal under-lava), which contain various golden items and items used to build portals such as obsidian and flint and steel.

Any ruined portal generated in the Overworld is surrounded with structures made of stone, stone bricks, and iron bars; in the Nether it is surrounded by blackstone variants and chains.

Currently there are 13 variants: 10 normal size portals and 3 giant portals in varying states of decay.

$$, ruined portals can be generated by the player by loading or  with a structure block. These ruined portals generate as they are stored, meaning they are not modified as detailed in the Generation section above. All portals generated this way create air pockets if generated in other blocks.

Portals
Note that some blocks are replaced with others upon generation. A full list can be seen above.

Trivia

 * Using the partially buried generation, some ruined portal variants are short enough that they can generate completely on the surface.
 * Ruined portals are the only generated structure that can be found in more than one dimension.
 * All ruined portal structures contain enough obsidian to make a complete portal. However, in basic portals, there is often not enough as each obsidian has a 15% chance to be replaced with crying obsidian.
 * Due to generating with magma blocks, underwater ruined portals create downward bubble columns.

Being Smart
you can rebuild or replace to fix the Nether portal and get to the overworld.