“This thing is like an onion,” George Costanza once said on “Seinfeld.” “The more layers you peel, the more it stinks.” Just about anyone who examines the Iran nuclear deal knows how he feels.

Look at the recent bombshell report that revealed Iran will be allowed to use its own officials to investigate a military site where it’s suspected of conducting nuclear weapons work, under the terms of a secret agreement it signed with the International Atomic Energy Agency.

This unprecedented arrangement has stoked concerns that the IAEA investigation of Iran’s past work on developing nuclear warheads will amount to little more than a public relations exercise.

The IAEA long has suspected that Iranian scientists experimented with high-explosive detonators for nuclear warheads at the Parchin military complex and conducted additional weapons-related work at other sites.

Despite repeated promises to cooperate fully, Iran since 2005 has blocked IAEA inspectors from looking at the Parchin facilities. It has also razed buildings and stripped away large quantities of earth, further fueling suspicions that Tehran is concealing evidence of past nuclear weapons work.

Why has the IAEA investigation, which began long before the Obama administration’s nuclear deal with Iran was signed, become a crucial part of the deal? Because the accord stipulates that sanctions can only be lifted after Tehran resolves the IAEA’s concerns over the “possible military dimensions” of its nuclear program.

The disturbing news that the IAEA has agreed to outsource some of its responsibilities to Iran has amplified concerns that questions about Iran’s past efforts to develop nuclear warheads will be swept under the rug in a rush to lift sanctions.

The IAEA’s absurd arrangement with Iran is a far cry from “anytime, anywhere” inspections the Obama administration promised. The final agreement reached allows Iran to delay inspections for up to 24 days, and possibly a lot longer if the U.N. Security Council gets involved in deliberations over possible Iranian efforts to delay inspections.

The disclosure of the IAEA secret agreement has ignited a firestorm in Congress. Sens. Mark Kirk, R-Ill., and Marco Rubio, R-Fla., have called for the administration to release secret letters to foreign governments assuring them that they will not be legally penalized for doing business with the Iranian government.

The IAEA’s ludicrous concessions, allowing Iranian personnel to gather possible evidence about past Iranian nuclear weapons experiments, set a dangerous precedent for future nuclear inspections. Letting Iran assume such a prominent role in investigating itself would amount to putting the fox in charge of the chicken coop.

Whether any more surprises lurk in other secret side deals is anyone’s guess. But it’s clear that the administration has oversold the deal.

President Barack Obama promised that “every pathway to a nuclear weapon is cut off.” But after 15 years, Iran will be free to ramp up its uranium enrichment efforts, which will make a sprint to a nuclear breakout much easier. Moreover, the deal will only hold if Iran complies with its nonproliferation commitments, which it has violated in the past.

The White House appears to be operating according to another George Costanza maxim: “It’s not a lie if you believe it.”