The key is a special liquid or powder that is poured or splashed onto the door, or poured into an opening of the door.

The door then dissolves or disappears, using the liquid/powder component in the process. Since the door has dissolved, it stays open.

The liquid could be a variety of special substances that would react with the door to cause its dissolution. The liquid could range from easily available, to incredibly difficult, and could force someone to search in a specific area (like a black dragons cave). For example

• There are many different types of acids that literally melt things. You could use scientific acids (hydroclhoric, sulfuric, etc), or something a little more fantastical like the acid from a black dragon's breath weapon.
• If the doors were constructed of something like salt of sugar, then almost any liquid would cause them to dissolve.
• The liquid could contain microorganisms (bacteria for example) that eat away at the material of the door. Similar to the extraterrestrial bacteria in The Andromeda Strain.
• In a sci-fi setting, the liquid could be nanobots that eat the door or convert it to a different shape.
• Thermite makes a very good door opening agent. The components of the door could cause an exothermic reaction, igniting the thermite, and melting the door.
• Salt or sugar, or some powdery concoction, could cause the door to melt when applied.

The key is a special liquid or powder that is poured or splashed onto the door, or poured into an opening of the door.

The door then dissolves or disappears, using the liquid/powder component in the process. Since the door has dissolved, it stays open.

The liquid could be a variety of special substances that would react with the door to cause its dissolution. The liquid could range from easily available, to incredibly difficult, and could force someone to search in a specific area (like a black dragons cave). For example

• There are many different types of acids that literally melt things. You could use scientific acids (hydrochloric, sulfuric, etc), or something a little more fantastical like the acid from a black dragon's breath weapon.
• If the doors were constructed of something like salt of sugar, then almost any liquid would cause them to dissolve.
• The liquid could contain microorganisms (bacteria for example) that eat away at the material of the door. Similar to the extraterrestrial bacteria in The Andromeda Strain.
• In a sci-fi setting, the liquid could be nanobots that eat the door or convert it to a different shape.
• Thermite makes a very good door opening agent. The components of the door could cause an exothermic reaction, igniting the thermite, and melting the door.
• Salt or sugar, or some powdery concoction, could cause the door to melt when applied. In Supernatural, we know that salt affects the demonic and supernatural, so if the door is made with demon flesh, it would surely melt when salt or saltwater (aka holy water) is applied to it.