Right now, the images suggest something is indeed underfoot.
According to the newest images that have been released to the public, activity continues near tunnel entrances at the northeastern mountain testing site of Punggye-ri, where North Korea has conducted three nuclear tests since 2006, the latest in February 2013.
Experts believe the country has developed a handful of crude nuclear devices and is working toward building a warhead small enough to mount on a long-range missile, although most experts say that goal may take years to achieve.
Among other signs of preparation, the imagery by a commercial Digital Globe satellite analyzed and released Wednesday by the Institute for Science and International Security, showed the presence of three black vehicles and a lighter-colored vehicle or truck on the small road leading directly to the South Portal tunnel entrance. It said that may suggest a VIP visited the site on April 29.
"All these activities are consistent with the view that a test or tests will occur soon, as has been reported in South Korean media," it said. "However, the exact timing of a test or tests is difficult to construe from the new activity.
It is also entirely possible that all the activity is a ruse. The North has been known to do that, too.
The heightened concern over a test follows warnings from Pyongyang that one might be in the offing, frequent claims by government officials recently of the North's sovereign right to have a nuclear deterrent and statements by the North that its nukes are a "treasure" and not "a political bargaining chip."
Pyongyang has gone into overdrive in its verbal attacks on Obama and South Korea's President Park Geun-hye since their summit last week. It has accused Washington and Seoul of pushing the Korean Peninsula to the brink of war with plots to attack their country and referred repeatedly to the North's claim that it has no choice but to develop a nuclear arsenal to protect itself against Washington.
"The DPRK will advance along the road of bolstering up (its) nuclear deterrent, unhindered, now that the U.S. brings the dark clouds of a nuclear war to hang over the DPRK," an unnamed spokesman of the North's Foreign Ministry was quoted by the state media as saying in a statement summing up Pyongyang's reaction to Obama's trip.
North Korea's official name is the Democratic People's Republic of Korea, or DPRK.
But the statement also said that although the test didn't occur while Obama was in Seoul, it could carry one out at any time.
"There is no statute of limitations to the DPRK's declaration that it will not rule out a new form of nuclear test," it said. "This is the exercise of the inviolable right to self-defense."
Regarding what Pyongyang might mean by a new kind of test, Lim Eul Chul, a North Korea expert at South Korea's Kyungnam University, said it could simultaneously conduct two nuclear blasts in two underground tunnels to show off its capability.
Another possibility: North Korea might try to detonate miniaturized forms of either plutonium- or uranium-based bombs, said nuclear expert Whang Joo-ho of Kyung Hee University in South Korea.
The warnings come amid rising tensions on the Korean Peninsula. Pyongyang strongly objected to recent annual joint military exercises by the U.S. and South Korea, and last month it test-fired two medium-range ballistic missiles and exchanged artillery fire with South Korea at their sea border.
During Obama's visit, Park said the assessment of her government was that North Korea is "fully ready now" to conduct another nuclear test.
The rhetoric about how the North's nuclear weapons are intended solely for defensive, deterrent purposes could be seen an as pre-emptive strike on the international criticism Pyongyang knows a new test would bring.
Another test explosion would deepen international concern about the North's development of weapons of mass destruction, and doubtless anger and embarrass the North's only major ally, China. Washington and its allies would push to tighten U.N. sanctions against Pyongyang.
While in Seoul, Obama said threats by North Korea will get it "nothing except further isolation" from the global community. But Obama acknowledged there are limits to what impacts additional penalties can have on the country.
"North Korea already is the most isolated country in the world by far," Obama said. "Its people suffer terribly because of the decisions its leaders have made. And we are not going to find a magic bullet that solves this problem overnight."
Lim at Kyungnam University said North Korea won't likely face tougher U.N. sanctions even if it conducts a fourth nuclear test because of strained ties between the U.S. and Russia, both veto-wielding members of the U.N. Security Council, over the Ukraine issue. He added that China, also embroiled in disputes with the U.S. over Japan, won't support tougher sanctions on North Korea, though it might agree on some form of punishment.