The Freedom From Religion Foundation is one of the country’s most dogged atheist legal groups. Answers in Genesis is one of the country’s most determined religious institutions. They’ve tangled a few times before, but a new storm is brewing, and the only ark we’ve got is the problem instead of the solution.

You may be familiar with Answers in Genesis’ Ark Encounter and Creation Museum — twin attractions that serve as a cross between a theme park, a Bible study and a tangible argument for six-day creation theory. The Creation Museum walks you through various perceived flaws in the theory of evolution, while the Ark Encounter pitches itself as an exact replica of Noah’s famed watercraft, give or take a few government-mandated exit signs.

Answers in Genesis is run by Ken Ham, and he’s drawn the ire of Freedom From Religion Foundation for encouraging public schools to plan field trips to his attractions in Kentucky. Atheist groups say that runs afoul of the First Amendment’s Establishment Clause. “Public schools and public school staff may not constitutionally organize trips to the Ark Encounter or the Creation Museum or any other religious venue,” says a letter the FFRF sent to over a thousand public schools.

“Ham has been clear about the proselytizing nature of this park from the beginning,” says the letter. “Though Ham asserts that the law is on his side, this is untrue. Unquestionably, any field trip facilitated by a public school to either attraction would be unconstitutional.”

Ham pushed back in a blog post, saying admission is free for public schools and arguing that since the Bible can be taught in the classroom as an objective part of human history, his theme parks are fair game as well.

“As leading civil rights attorneys will tell you,” he wrote, “if classes tour the Ark or museum in an objective fashion to supplement the teaching of world religions, literature, interpretation of history, etc., the field trip is an educational experience. Now, if students were brought to the Ark or museum and told by their teacher that the religious content should be accepted as truth, then we would acknowledge that the Establishment Clause of the Constitution, as currently being interpreted by the courts, would be violated.”

Ham cautioned the FFRF against harassing schools and called them “atheist bullies …who have been wreaking havoc on civil liberties all across America.” He said he didn’t fear a legal challenge.