The mysterious ruins are teleportation hubs which connect the pocket dimensions of the Runecrafting altars to the physical world of Gielinor. Players can use talismans on the altars, wear a tiara or wield a talisman staff to be teleported into the pocket dimension.
As with all teleportation this occurs via the Abyss. Indeed, direct usage of the abyss allows players to bypass the mysterious ruins to access the altars.
In general the mysterious ruins are located in a place where the environment matches the element of the Runecrafting altar. This is likely to be the result of the energy of the altar (which warps space to create the pocket dimension) seeping out and influencing the environment.
Runecrafting teleport tablets allow players to teleport next to the mysterious ruins for that element (the player will still need the required item to enter the altar).