My regular group is looking to possibly travel to the Elemental Plane of Earth for some metal samples.  They are looking to travel using an Amulet of the Planes.  I'm making them research locations, but the real question is:

What happens to them when the arrive there?

Time was, characters needed certain magic (spells, items, etc.) to survive in the elemental planes. (I also have a vague recollection of instant death for the Elemental Plane of Fire unless you were protected.)  The only thing I can think of that can assist them immediately is the Necklace of Adaptation. 

But is this even necessary anymore? 

Is there some rule, note, book, etc. that elaborates on if characters need such protections?

TIA!

Unfortunately what information there is about planes is limited and scattered in 5e because they haven't made a planescape setting book or guide to the planes or anything.

There is still a pretty high chance of death in most planes without precautions (drowning is water, falling forever in air, I desert literally made of fire). Earth should be one of the safer ones.

The two biggest risks for mortals visiting the Elemental Plane of Earth are being entombed, and the general lack of illumination.  The landscape in the Elemental Planes is more fluid than it is in the Prime Material World, so there's always a risk of a cave-in or simply your tunnel shifting and burying you alive.  Earth elementals don't notice this since they can generally tunnel through the earth unimpeded when this happens, but it tends to be lethal for unprepared mortals.  When adventuring in the Plane of Earth, a trustworthy local guide is a must-have.

Similarly, there's often no sources of light outside of cities, so if a character doesn't have darkvision make sure they have a nice permanent source of light that doesn't use up oxygen (in other words, something magic rather than a torch or lantern) or a magic item that grants darkvision.

Though the truly biggest threat on the plane could just be that Dao like enslaving any stray mortals they find.

Check out Princes of the Apocalypse for some planar context.

Depending on your needs, the planes can either be uniformly hazardous, or have borders akin to the border ethereal, where it is merely uncomfortable, rather than insufferable. The "deeper" you go, the more extreme the planar effects become. On the Elemental Plane of Fire, I believe that includes a Firestorm so hot that it can incinerate creatures that are otherwise immune to fire damage.

For the Elemental Plane of Earth, you can have them appear safely in underground tunnels, but subject them to appropriate hazards, such as regular earthquakes, cave-ins, and all manner of seismic shifting. The longer they stay, the more likely they are to get trapped in a pocket of earth unless they have a burrow speed, or bring along a few giant badgers to keep them moving forward. Since it is a Plane of Earth, you could easily justify massive open air complexes that fit entire cities.

Note that an "air pocket" doesn't necessarily mean that the air is breathable. Such places can be filled with carbon monoxide, hydrogen sulfide, or any number of terrestrial dangers.

Thanks for the info.  One of the characters is a Druid, so they'll burrow.  They are also considering getting a Necklace of adaptation in case things get dicey.

The "deeper" you go, the more extreme the planar effects become.

This was my impression of how they did the elemental planes in 5e. You basically have a gradient that goes from 'pretty much the regular world with a little more elemental flair' all the way to churning, chaotic destruction. Thus you can decide how inhospitable you want it to be based on where you set the adventure .

Water is said to be the most habitable elemental plane

Quote from Nuker_period707935b87e >>

Water is said to be the most habitable elemental plane

 Only if you're a fish or can otherwise breathe underwater.  If you can't, then the Elemental Plane of Air is the most inhabitable one, even if you can't fly.