Well, osmium is your friend, and a little less radioactive IIRC. (I have a vague recollection of the kids on Chocky playing with a ball of osmium which, frankly, theres no way they would have been able to lift - but at least theyd have been sterilised by the radioactivity, so given how annoying they were theres an upside.)

Not that Ive yet found many roboteers looking for ballast...

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Fluppet