Swimming in acid takes one ability, Isma's Tear, tucked away in the Royal Waterways behind two other unlocks.
Acid pools in Hollow Knight kill you on contact until you find the right ability. Per the wiki, the only way to swim in acid safely is Isma's Tear, an ability whose tagline reads "Acid shall be repelled."
According to the wiki, Isma's Tear lets the Knight swim in acidic water without taking any damage. Before you have it, every acid pool is an instant hazard that knocks you back and drains a mask.
This guide explains exactly where to find Isma's Tear, the two unlocks that gate it, a clear route to reach it, and everything the ability opens up once acid stops being deadly.
Get Isma's Tear to swim in acid without taking damage.
Isma's Grove, deep in the Royal Waterways under the City.
The Crystal Heart super-dash is needed to reach the grove.
Beat the Dung Defender to open the path into the Waterways.Per the wiki, the ability is Isma's Tear, and its in-game description is simply "Swim in acidic waters without coming to any harm." Once you have it, acid behaves like ordinary water.
According to the wiki, Isma's Tear is named for 
Isma, one of the Five Great Knights, whose grove the ability is found in. The tear is the last trace of her in the flooded ruins below the kingdom.
The wiki notes that Isma's Tear is not strictly required to finish the game. In practice, though, it removes a constant hazard and opens so many shortcuts that most players grab it as soon as they can.
Per the wiki, Isma's Tear sits in Isma's Grove, a small green chamber within the Royal Waterways. The Waterways are the sewer system beneath the 
City of Tears.
According to the wiki, the grove is one of the few living, leafy spots down there, marked by a single glowing pickup. Reaching it means wading through acid channels, which is the catch, since you do not yet have the ability that makes acid safe.
Isma's Grove also holds a Grub. Free it while you are there so you do not have to wade back through the Waterways a second time for the Grubfather reward.
Per the wiki, two unlocks gate the grove. You need both before Isma's Tear is reachable on the normal route.
| Requirement | Why it is needed |
|---|---|
| Crystal Heart | The super-dash carries you across the long gap into the grove |
| Dung Defender | Beating this boss opens the path down into the Royal Waterways |
According to the wiki, there is a far harder alternative: dash, pogo-bounce off the spikes, then immediately dash again. The wiki calls this almost impossible without frame-perfect timing, so the Crystal Heart route is the one to use.
The wiki notes that the Crystal Heart comes from 
Crystal Peak, while the Dung Defender waits in the Waterways itself. In real runs, most players already have the Crystal Heart by the time they think to explore the sewers.
City of Tears, in the flooded rooms near the bottom of the city.
Grub in the grove on your way out.Per the wiki, with acid no longer a threat, several previously blocked areas and pickups become reachable.
The Shape of Unn charm in 
Greenpath.
Easier access to Fog Canyon through the Crossroads or Fungal Wastes, plus the Uumuu fight.
A route into the Queen's Gardens.
A Grub and a shortcut in Kingdom's Edge.According to the wiki, Isma's Tear is also handy while fighting 
Uumuu in Fog Canyon. In actual play, the ability quietly turns dead-end acid rooms all over Hallownest into open routes.
Isma's Tear connects to the acid-blocked routes it opens and the fast travel that gets you back to them. These spokes pick up the connected threads.
Game data and screenshots adapted from hollowknight.fandom.com, used under CC BY-SA 3.0. Original content remains the property of the wiki contributors and Team Cherry.