The world watched uneasily on Feb. 24 when Russian troops, under the orders of President Vladimir Putin, invaded Ukraine.
"The prospect of nuclear conflict, once unthinkable, is now back within the realm of possibility," U.N. Secretary-General Antonio Guterres said March 14.
Nearly 70% of Americans surveyed by the American Psychological Association said they "are worried the invasion of Ukraine is going to lead to nuclear war, and that they fear that we are at the beginning stages of World War III."
Researchers estimate there are approximately 12,700 nuclear weapons spread between nine countries, with the United States and Russia holding the majority.
Researchers and government officials stress that a nuclear attack is very unlikely.
"We are assessing President Putin's directive and, at this time, we see no reason to change our own alert levels," White House press secretary Jen Psaki said Feb. 28.
"As long as these weapons are around, the possibility will always be there that they will actually be used," said Alejandra Munoz, a project officer at the Dutch peace organization PAX.
Researchers, academics and advocacy groups for risk reduction and nuclear disarmament have written reports detailing what a hypothetical attack could look like and the long-term impact it could have on the planet and society.
Here's what a nuclear attack could look like, hypothetically, and how the United States might respond.