Overnight there were massive explosions at multiple military bases in Syria, including bases believed to be run by Iran.

One of the explosions, which was a combination of enormous secondary explosions, was so large it registered as a low-level earthquake on seismic instruments in neighboring countries.

https://www.emsc-csem.org/Earthquake/earthquake.php?id=662356

Here is video of the explosions at one base:

The working assumption is that Israel hit storage areas where Iran kept heavy long-range rockets for future use against Israel. The NY Times reports on conflicting reports as to the death toll and how many of the dead were Iranian:

Missile strikes in Syria hit military bases used by Iran and its proxy militias fighting in the civil war there, killing at least 16 people, a conflict monitoring group and one of Iran’s regional allies said on Monday.

The Syrian state news media confirmed that there were strikes in the central city of Hama and near the northern city of Aleppo late Sunday, but it did not provide a death toll. An official for the regional alliance that includes Iran, Syria and the militant group Hezbollah said that most of the dead were Iranian….

The Syrian Observatory for Human Rights, which is based in Britain but monitors Syria through contacts there, said that the attacks on Sunday had killed at least 26 people, many of them Iranians.

The strikes hit munitions warehouses at the 47th Brigade military base near Hama that Iran and its proxies used, destroying ground-to-ground missiles and causing large explosions, the observatory said. Another attack struck the Neirab military air base near Aleppo.

An official from a regional alliance that includes Iran, Syria and Hezbollah confirmed that the strikes near Hama had hit a storage facility on the base and said they had destroyed 200 missiles and killed 16 people, including 11 Iranians. The official spoke on the condition of anonymity because he was not authorized to speak to journalists.

Haaretz adds that Arab media report bunker buster bombs were used:

The targeted bases were manned by the Syrian army as well as Iran’s Revolutionary Guard, and housed several missile warehouses, according to the Lebanese newspaper Al Akhbar. The report adds that several dozen were wounded and evacuated to nearby hospitals. According to the report, Bunker Buster bombs were used, which makes it likely the launch was airborne.

Avi Issacharoff at The Times of Israel suggests that the seismic activity may reflect the use of bunker busters, rather than the secondary explosions, but that the real significance is the size of the attack and that is took place just after Secretary of State Mike Pompeo visited Israel:

An increasing number of media organizations associated with the Syrian regime and Hezbollah are hinting that Israel was responsible. According to a report in the Al Akhbar newspaper, identified with Hezbollah, bunker buster missiles, which do not explode on impact but rather deep in the ground, hit bases in the Hama and Aleppo areas. Hence the “earthquake.” …

Whatever the case, it is clear that the strike was highly unusual in several respects.

First and foremost was the sheer power of the attack. The pictures and the sounds, and the large number of casualties, point to an incident of larger scale than those to which we have become accustomed. We are not talking here about just another strike on another Hezbollah convoy, but rather what would appear to be a new step in what is now the almost-open warfare being waged between Iran and Israel in recent weeks on Syrian territory. The same player that earlier this month attacked the T-4 airbase, from which an Iranian attack drone was launched into Israel in February, apparently struck again overnight Sunday-Monday, taking the gloves off and moving into a new level of military confrontation….

All this is beginning to look rather like a coordinated Israeli-American operation to limit Iran’s military activities in Syria — simultaneously conveying the message to Moscow that Russia’s green light for Iran to establish itself militarily in Syria is not acceptable in Jerusalem and Washington.

Assuming Israel was behind the explosions, it shows Israel’s willingness to confront Iran directly in Syria. Iran has vowed revenge for an earlier strike at an Iranian drone airbase in Syria, but if these current reports are correct, the Iranian death toll was much higher this time. By striking missile depots with the types of weapons Iran likely would fire from Syria, Israel may have been acting to preempt retaliation.

This is not the best time for Iran to try to attack Israel, with the Trump administration decision on the Iran nuclear deal just weeks away. Moreover, Iran would be fighting far from its territory, in an area where Israel has shown it controls the skies. The Russians seem to be stepping aside, for now, and not trying to interfere with Israeli attacks on Iranian positions in Iran. So large missiles fired from Syria were and remain Iran’s best way of attacking Israel. Now at least some of those missiles have done up in flames.

We will update this post as more becomes known.