Hidden in Gaza, or in the network of tunnels that run beneath it, are upwards of 100 hostages.
Soldiers, women, children, the elderly, taken from their homes, snatched from a music festival.
With so many people held in an under-siege war zone, experts told Sky News it was one of the most complex hostage situations they have seen.
Now governments and negotiators have to try and get the hostages out alive.
Former hostage negotiator Scott Walker told Sky News the first task of a negotiator is to get alongside the family of the victim.
They are normally the point of contact for the kidnappers, so working out who will speak and what will be said is crucial.
Initial calls are to “reassure the hostage takers that we take them seriously” and reinforce the importance of treating the hostages well.
The first big challenge now will be finding out where in Gaza the hostages are and working out exactly who is holding them, he said.
“Is this a big coordinated effort by Hamas, or are there little sub-groups all operating on their own with their own potential communication lines to the families or to the third-party intermediaries?”
With tensions between Hamas and Israel “at boiling point”, the people trying to get the hostages out safely have “got a challenge on their hands”, former head of Hostage US Rachel Briggs told Sky News.
“To resolve a case like this, you’ve got to have the willingness of both sides to get to the table in the first place,” she said – and that willingness is vanishingly small.
What is happening to hostages
For the hostages this is “psychologically and physically one of the toughest situations you could imagine”, Ms Briggs said
“Psychologically it’s absolutely terrifying. Not only are they being held hostage … but they’re being held hostage in the middle of a war zone.
“Physically many of them may have arrived in captivity in not great physical shape, potentially injured and possibly some seriously injured.”
Footage from Hamas’s surprise attack on Saturday morning showed some hostages bleeding as they were bundled away.
What is happening to the hostages “depends on the reasons why Hamas has taken them”, Mr Walker said.
“Clearly, they’re being used as an element of a human shield right now. And ultimately, one of the main reasons potentially is prisoner exchange.”
Chances of a rescue
Those held captive will be wishing for a speedy rescue, but rescue will be “the option of last resort”, Ms Briggs told Sky News.
“It is so dangerous and it’s so uncertain,” she said.
Governments will be drawing up rescue plans to make sure every option is available to them – but they rely on having a high level of certainty about where the hostages are and whether they can safely get them out without losing any personnel.
Hostage rescues happen “relatively rarely” because they are so high risk, Ms Briggs said.
“It can end badly as often as it ends well.”
How long could people be kept captive?
In such a fast-moving and highly unpredictable situation, both experts agreed it was impossible to predict how long the hostage situation could last.
Mr Walker said Hamas may agree to the release of some women, children and older people in order to “retain what support it has in some quarters”.
“They’re unlikely to release any Israeli military hostages because their bargaining value will be higher,” he said.
But Hamas also has a track record for holding hostages for long periods – notably Gilad Shalit, the Israeli soldier held for five years and eventually released in return for more than 1,000 Palestinian prisoners.
A small comfort is that “the Israelis really do have a policy of never leaving anyone behind”, Ms Briggs said.
“The fact that they are drawing around themselves expertise such as the Americans and other allies, I have no doubt that this will be a priority for them, even if what we’re seeing at the moment is dominated by a different set of activities.”
Help from overseas
The US has sent technical experts to assist in hostage recovery and Mr Walker said he understood the Israelis had set up a hostage situation coordination cell to manage the response.
The hostages are mostly Israeli but it is thought other nationalities including Britons, Americans, Germans, Thais, Mexicans, Brazilians and Nepalis are among their number.
Qatar said it was in talks with Hamas to swap Israeli women and children for Palestinian prisoners. While Israel denied such negotiations were taking place, Ms Briggs said the news from Qatar was a positive development.
“Qatar has a very long track record in hostage cases of playing a quiet and helpful role behind the scenes helping to get folks out of some very difficult situations,” she said.
“It’s not necessarily impossible that things can be moving in that direction right now.”
Families of the captured have begged for their release.
While the situation is “perilous”, Ms Briggs said it was vital to “maintain hope”.
“That’s not a case of wishful thinking,” she said.
“I’ve seen dozens and dozens and dozens of cases over the last 20 years, and I have seen hostages walk out of all kinds of situations that you never thought they would manage to get out of alive. It does happen.”