The vaguest hint is that EVERYTHING you need to know is in the Achievement Title and Description. The image provides no guidance.

Title: SIGHT, SEA, LIGHT
Description: DEATH, DEATH, DEATH

Very simple Title Hints:
Sight = SPECIFIC Item
Sea = GENERAL area
Light = SPECIFIC spot

Very simple Desc Hint:
Death, = Die
Death, = Die
Death = Die

Title Hints:
SIGHT = Find a tower with eyes at the peak. Don't forget to bring your own Eye.

SEA = Find the rift at the edge,

LIGHT = Climb to the LIGHT

Description Hints:
Death 3x - very very very self explanatory. Check below if you want to know specifically HOW.

MASSIVE SPOILERS BELOW. I piecemealed the information so you can choose to get deeper and deeper into the spoilers and stop when you think you want to figure stuff out for yourself. Sight, Sea, and Light hints are self-referential only and point to one of the other three element hints for details, so you can navigate around each of the three clues independently.

Further explained Title Hint:
SIGHT = You need to FIRST beat the campaign as it currently is. This unlocks the Endless Substructure mode. You don't have to cycle all the way back to the Silo section, but you do have to get to at LEAST the 1500m point.

More SIGHT guidance: 1500m is the start of the "Lost Pier" zone which is UNIQUE TO ENDLESS SUBSTRUCTURE. There will be a BLINK EYE item on a dumpster right at the start of Lost Pier. Use the Blink Eye twice to unlock it in the campaign vending machines.

Need to know what to do with it? Find the blink eye in a vending machine in the Campaign and buy it. Bring it to the 'SEA,' and use it at the 'LIGHT.' This will only work if you have fulfilled the condition explicitly described in the Description of the achievement
- Desc explanation below if you need help.

SEA = In the Campaign, get to the Haunted Pier. There are 2+ variations of Haunted Pier. The ONLY VARIANT you can OPEN the rift on is the one with the radio mast at the end of a long truss bridge. The rift will only appear on the other variant once you have completed the achievement.

More SEA guidance: The rift only appears if you have died at least once. The rift is only active if you have died THREE times. Utilize the item implicated in SIGHT to enter the rift. See LIGHT if you need to know where

LIGHT = Stand ON the red blinking light at the top of the radio mast. Only one of the Haunted Pier variations will allow you to open the rift for the first time, but both variants have a Rift that will appear after you complete the achievement. You'll know where it is in the obvious variant when you see the giant tower connected to the Pier via a truss bridge. If you have fulfilled at least one of the 3 Description conditions, you will see a RIFT appear when you reach the top of the tower.

Further explained Desc Hint: First Interlude zone has a save terminal that sells you a 1-life Floppy Disc. The Second Interlude zone has a save terminal that sells you a 2-life Floppy Disc. You can also find floppy discs in secret areas that work. You can also get the DISC JOCKEY Experimental perk that gives you a 3-life golden floppy.

In case it's not obvious, you have to die 3+ times for the Rift to be active. Dying once or twice will make it appear, but you can't do anything with it.

WARNING: YOUR CURSOR IS ENTERING RISKY TERRITORY.

The spoiler text here reveals EVERYTHING at ONCE with PICTURES. Do not hover anywhere near this if you don't want to be visually spoiled.

This is NOT JUST TEXT. This is PICTURES.

You have been warned.

You are entering THE DANGER ZONE

Kenny Loggins, play us out...

Beat the campaign to unlock Endless Substructure. In this new mode, Get to 1500m, the start of the unique "Lost Pier" zone. On a dumpster right at the start of the zone, there will be a BLINK EYE item. Use the BLINK EYE two times, and it will now start appearing in Campaign vending machines.




Go back into the campaign and navigate to the Haunted Pier section of Habitation-Yellow. Make sure to buy a Blink Eye if you find it. You will also have to die 3 times to actually activate the rift where you need to use the Eye.

Some perks help with this goal, like Disc Jockey from the experimental perk computer. The Golden Disk from this perk grants 3 extra lives, the exact number you need to die with. Since the Experimental perk machine is in the Silo zone, before the first Interlude, you can carry this straight there and die three times, and STILL purchase the other saves like normal for backup lives. (I do not currently know if dying 4+ times negates the Rift from appearing...) Also, since you lose your items on death, the 10-bug ANOMALOUS BONDS perk is VERY handy so you can keep the Blink Eye if you bought it in a zone before the first or second Interludes. ANOMALOUS BONDS lets you keep your items upon reviving.


(Image Credit kittyfan01 on Discord)

Once you have a BLINK EYE and have died three times, get past the first and second interludes, get past the Tooth monster, and make it to the Haunted Pier. There are 2+ variations on Haunted Pier, the one you want is the one with the giant tower at the end of a truss bridge. If you don't have this (you'll know once you see it) then you have to restart.

You will be able to do this on either variant of Haunted Pier ONLY AFTER YOU GET THE ACHIEVEMENT. If you want to know where the Rift appears afterwards in the second variant, go to the far end of the pole with the Red blinking light - see below image.

^ The pole. Note, the Rift will only appear here once you have opened the rift on the Radio Mast variant.

When on the Red blinking light. Look at the rift, and use the blink eye. This will teleport you into a new area, giving you the achievement, and allowing the rift to appear on all Haunted Pier variants.

Congratulations.

NOTE: In subsequent runs, you will not need to die 3 times. The Rift will always be open, provided you have the Blink Eye to enter the rift. This is expected to be used as a shortcut to later levels of the game. I won't spoil what is inside the rift that could be used for this, though. I will let you have SOME fun discovery. Besides, that's not part of any achievement!

User justSnowy has provided a very thorough video tutorial that you can follow. This hand-holds you through the whole process, and is very succinct. This video does NOT give vague or leading hints, it explains it to you OUTRIGHT. Do not watch if you want a chance to solve it yourself.