The list is, officially, eighteen positions long, illustrated here (with a typo).
- Vice President
- Speaker of the House of Representatives
- President Pro Tempore of the Senate
- Secretary of State
- Secretary of the Treasury
- Secretary of Defense
- Attorney General
- Secretary of the Interior
- Secretary of Agriculture
- Secretary of Commerce
- Secretary of Labor
- Secretary of Health and Human Services
- Secretary of Housing and Urban Development
- Secretary of Transportation
- Secretary of Energy
- Secretary of Education
- Secretary of Veterans Affairs
- Secretary of Homeland Security
You are correct that we've never gone past the VP, which we've only gotten to nine times.
It's important to note that the depth of the presidential 'bench,' if you will, varies because the requirements to become President are stricter than those necessary to become, e.g. Speaker of the House. As such, there may be people in the statutory line of succession that would be skipped.
You can't even inherit the duties of the job if you couldn't hold the job itself. So this holds even though no one below the VP actually becomes the President, they simply assume the duties and responsibilities thereof.
Per 3 U.S. Code § 19(a)(1) emphasis added:
If, by reason of death, resignation, removal from office, inability, or failure to qualify, there is neither a President nor Vice President to discharge the powers and duties of the office of President, then the Speaker of the House of Representatives shall, upon his resignation as Speaker and as Representative in Congress, act as President.
Legally the term 'act as President' is decidedly and explicitly not the same as 'becomes the President.' And later on in that same part of the USC it's explained that if at any point the President or Vice President become competent to discharge the duties of their office, the successor is dismissed - this is NOT true, however, if someone higher up the chain - but not the President or VP - becomes competent. Irrelevant in cases of death, but in the case of sickness or kidnapping, it would be possible for someone down the chain to need to assume the duties until the crisis was resolved.