Idiomatic expression or word for a very heavy thing

There is an idiomatic expression in Turkish for describing very heavy things: "Heavy like a dead donkey" (Eşek ölüsü gibi ağır).

In English, there is a dead weight but it is not quite similar. Also, it has two meanings, the first of which is related to heavy weight but used for people in certain contexts. I'm not sure if it used for objects also. And the usage of the second meaning seems like more common. (It might need analysis).

1. if someone is a dead weight, they are very heavy and difficult to carry, often because they are not conscious. Tom was a dead weight and her muscles ached as she carried him upstairs.
2. something or someone who prevents other people from making progress We must free ourselves from the dead weight of history. She's just a dead weight on the business at the moment.
_{Source: http://idioms.thefreedictionary.com/a+dead+weight}

I think there is a misconception of dead being heavier than alive and it is reflected to languages also but technically it is not correct. A dead body has the same weight as a live body. The misconception comes from the fact that a live (and conscious) person can distribute weight and adjust the center of gravity for a better balance. But, "dead weight" is the center of mass being wherever supported, lacking any effort on their part, and therefore concentrated in one spot, more or less.

Why am I saying all of these? Because there is too much technicality and different senses behind a dead weight.

Is there any word, idiom or other fixed phrase to place in a natural-sounding sentence? As in:

• heavy like a __________

or

• as heavy as a __________

For example:

The box was heavy like a ________; I couldn't carry it by myself.

Are these usages flexible enough to allow the slot to be filled with different words that convey heaviness?

Other than that, is there a single word that means "very heavy" or "extremely heavy" ?

The most common idiom for something heavy is not a single word. You will hear it commonly when people are trying to lift heavy things:

This weighs a ton!

It's really more of a hyperbole than an idiom, but it is what's said.

I think that a known expression to refer to something very heavy is:

As heavy as lead

Ngram shows that its is usage has been decreasing in last decades but it is still used.

Lead is known for its high specific gravity, which I think is the reason why it is used to refer to something very heavy.