Hi this is pokemaster796 I was wondering what is the easiest way to..

You can catch it in front of sootopolis by diving. Just catch it with an ultra ball, or wait a lot of turns and use the timer ball. Altering cave is where all these rare Pokemon are like: aipom houndour, and that's just to name a few. But you have to do something with the mystery gift to unlock the Pokemon there.

Altering cave is just a stupid cave after you beat the elite 4 it opens

Basicly it is just a cave with zubats in it

it is just a cave with zubat in it

A dumb cave with zubats

You get relicanth underwater in route 124-126 it's a rare find though and even rarer to find is a relicanth holding a green shard(exchange it for leaf stone a some remote island [like the Kame house from DBZ] near mossdeep city) but you can find one with that item.
Altering cave I beleive has no signifficance to the game whatsoever.

Relicanth are quite rare on Emerald, you can find them underwater on routes 124-126, it might be better to use a Dive Ball.
Oh and Altering Cave is a mysterious chamber that the people used to seal Regice, Registeel and Regirock away because of their power. To get to the cave you will need Dig and Surf. Keep in mind that you need the Braille chart in your Manual to translate the dots and find out what to do.
Surf east from Pacifilog, making sure to keep low down and don't miss the Dive area that you need to go through.
Go up and then you must have Relicanth at the start of your team and Wailord at the end. (Wailmer evolves at lv.40 to Wailord)
Read the rock and the ground will shake, the three chambers around Hoenn are now open. All regi's are lv. 40 and are quite powerful.
Their chambers:
Regirock is in the desert.
Registeel is left of the Safari Zone and up a hill.
Regice is on your way(surfing) to Dewford.

Hope I helped!

The easiest way to catch Relicanth is to go to were there is grass underwater and look for it.

Okay, first of all, make sure that Relicanth doesn't use yawn. Also, make sure that you know any move that puts the foe asleep, like sleep powder, hypnosis, sing, ect. And, last but not least, have a Pokemon that knows false swipe. A Pokemon that knows false swipe and (I think) sleep powder would be Sceptile. So, use false swipe so that he/she is in the red zone. Then use sleep powder with any Pokemon, but if Sceptile knows it that would be awesome. But, the point is, just use it. Then, keep throwing Ultra Balls. You'll catch him soon. As for Altering Cave, it is located on Route 103 after you beat the Elite Four. At first, you'll only find Zubat. But, you find other Pokemon by the use of Mystery Gift. Someone told me that Mystery Gift part, but I think they're right. Anyway, You'll only find Zubat, so it's a pretty useless place unless you use Mystery Gift (I think...) Hope I at least helped a little bit!

You can catch Relicanth around routes 124-126 on the ocean floor(got it from the official Pokemon emerald guide book).
All those ones who told you altering cave only has zubats in it lied to you. Upon me first entering it I found Mew at level 16. There are many of them there so go back in there and explore. that's why it's called ALTERING cave. Looks are deceiving. Hoped i've helped.