With the right equipment it is possible to probe the inside of a chip, allowing an attacker to measure or even alter internal signals down to the transistor level. Expensive, but very useful if it lets you extract a crucial shared secret.

The traditional defense against this kind of invasive attack is to put a grid of sense wires on the outermost metal layer, and measuring whether it has been tampered with: you can't get to the important bits without cutting through the security grid, but any kind of modification to the security grid triggers a self-destruct.