Something very, very strange happens one day, very suddenly. Four caves on four different worlds - their relative locations different, their composition similar but not identical, the connection between them beyond the newly formed one quite a mystery -

A light show surrounds and emerges from those caves, visible for miles, and assorted sounds roll out from their mouths, and the depths of them blip a bit, and elide into each other in vaguely plausible places that definitely didn't contain 'an entire other cave system' earlier. This forms an odd sort of middle room, a large space blurring the access needs and characteristics of all four planets, and several spaces tumbling off of that - the original cave systems themselves, of course, as well as a few other jagged meeting points.

On a planet the natives more or less agree should be called Kris* when talking to aliens about it, this anomaly opens up in within the rural outskirts of the most populous city-state in the world, in one of the few cave-clusters among the extensive systems surrounding the city not actually connected to any others. It's a bit awkward to get to, but not badly so, and the awkwardness makes it easier to cordon off.

The first responders message the city council, who're already initiating inbound travel stops and messaging the other cities around them (with a strong priority, of course, for those they have evacuation agreements with, and then for any city in the area who for whatever reason isn't party to those agreements, then for anyone who might have expertise or particularly broad ranging disaster response ability, just in case). The district manager coordinates with first response to keep the area clear. Several different redundant systems put assorted emergency responses into standby mode.

And a team enters the cave as soon as they've donned what they hope is the right protective gear, even before the light show ends. 

*There was significant additional debate about this for sending out extrastellar probes and not just theorizing about responding to alien transmissions, since 'Kris' is derived from a phrase that more or less means 'the place I'm speaking from.' The people who argued that you also use that phrase in letters, which an unmanned probe basically is, and it's often figurative anyways, eventually won this particular debate. (The debate still comes up of course, but it's more of a running joke nowadays.)