Since thermal energy is only lost at boundaries, the larger the amount you have, the smaller the net surface area per unit of volume. This is how you can store

Since thermal energy is only lost at boundaries, the larger the amount you have, the smaller the net surface area per unit of volume. This is how you can store huge blocks of ice well into the summer (e.g. https://en.wikipedia.org/wiki/Ice_house_(building) https://en.wikipedia.org/wiki/Ice_house_(building)), but also how a giant underground mass of molten salt that's not in physical contact with the surrounding ground could store a lot of heat for a long time.