Returning home to the Piaaraa after a long absence, let me take a look at the next line in the shabad of Guru Ram Das Dev ji(see Post 1).
ਆਪੇ ਗਹਿਰ ਗੰਭੀਰੁ ਹੈ ਪਿਆਰਾ ਤਿਸੁ ਜੇਵਡੁ ਅਵਰੁ ਨ ਕੋਇ ॥
aapae gehir ganbheer hai piaaraa this jaevadd avar n koe ||
The Beloved Himself is deep and profound and unfathomable; there is no other as great as He.
Guru Ram Das Dev ji is saying two things -- The Beloved is so vast that His nature is unfathomable-- it cannot be measured. But the word 'fathom' refers to measuring the depth of a great body of water, such as the ocean or a lake cut out of mountains by a receding glacier. It is the depth of the Beloved that cannot be understood, because it is unfathomable. It is profound, beyond the ability of our minds to comprehend or even imagine. Guru Ram Das is also saying that He is the greatest, that no one is greater in any dimension the human mind can think of.
How does his son Guru Arjan Dev ji respond? He says, in sri Raag (Ang 50),
ਸਤਿਗੁਰੁ ਗਹਿਰ ਗਭੀਰੁ ਹੈ ਸੁਖ ਸਾਗਰੁ ਅਘਖੰਡੁ ॥
sathigur gehir gabheer hai sukh saagar aghakhandd ||
The True Guru is the Deep and Profound Ocean of Peace, the Destroyer of sin.
ਜਿਨਿ ਗੁਰੁ ਸੇਵਿਆ ਆਪਣਾ ਜਮਦੂਤ ਨ ਲਾਗੈ ਡੰਡੁ ॥
jin gur saeviaa aapanaa jamadhooth n laagai ddandd ||
For those who serve their Guru, there is no punishment at the hands of the Messenger of Death.
ਗੁਰ ਨਾਲਿ ਤੁਲਿ ਨ ਲਗਈ ਖੋਜਿ ਡਿਠਾ ਬ੍ਰਹਮੰਡੁ ॥
gur naal thul n lagee khoj ddithaa brehamandd ||
There is none to compare with the Guru; I have searched and looked throughout the entire universe.
ਨਾਮੁ ਨਿਧਾਨੁ ਸਤਿਗੁਰਿ ਦੀਆ ਸੁਖੁ ਨਾਨਕ ਮਨ ਮਹਿ ਮੰਡੁ ॥੪॥੨੦॥੯੦॥
naam nidhhaan sathigur dheeaa sukh naanak man mehi mandd ||4||20||90||
The True Guru has bestowed the Treasure of the Naam, the Name of the Lord. O Nanak, the mind is filled with peace. ||4||20||90||
Guru Arjan Dev compares the Piaaraa to the ocean, an Ocean of Peace. It is this mental state of complete tranquility and quiet that is so profound to understand. But for those who serve Him, for those who eventually are able to grasp His depth, all sins are destroyed and the messenger of death can impose no punishment. This is mukhti. There is no beloved quite like Him, Guru Arjan Dev tells us. He has searched the entire universe as guru. What he has found, however, is that the True Guru, this Ocean of Peace, once attained, fills the mind with peace. Our souls are like the basin and the peaceful waters of the Divine fill this basin of our souls.
In raag majh, on Ang 99, Guru Arjan Dev repeats these themes,
ਮਾਝ ਮਹਲਾ ੫ ॥
maajh mehalaa 5 ||
4 ਲਾਲ ਗੋਪਾਲ ਦਇਆਲ ਰੰਗੀਲੇ ॥
laal gopaal dhaeiaal rangeelae ||
O my Love, Sustainer of the World, Merciful, Loving Lord,
ਹਿਰ ਗੰਭੀਰ ਬੇਅੰਤ ਗੋਵਿੰਦੇ ॥
gehir ganbheer baeanth govindhae ||
Profoundly Deep, Infinite Lord of the Universe,
ਊਚ ਅਥਾਹ ਬੇਅੰਤ ਸੁਆਮੀ ਸਿਮਰਿ ਸਿਮਰਿ ਹਉ ਜੀਵਾਂ ਜੀਉ ॥੧॥
ooch athhaah baeanth suaamee simar simar ho jeevaan jeeo ||1||
Highest of the High, Unfathomable, Infinite Lord and Master: continually remembering You in deep meditation, I live. ||1||
ਦੁਖ ਭੰਜਨ ਨਿਧਾਨ ਅਮੋਲੇ ॥
dhukh bhanjan nidhhaan amolae ||
O Destroyer of pain, Priceless Treasure,
ਨਿਰਭਉ ਨਿਰਵੈਰ ਅਥਾਹ ਅਤੋਲੇ ॥
nirabho niravair athhaah atholae ||
Fearless, free of hate, Unfathomable, Immeasurable,
ਅਕਾਲ ਮੂਰਤਿ ਅਜੂਨੀ ਸੰਭੌ ਮਨ ਸਿਮਰਤ ਠੰਢਾ ਥੀਵਾਂ ਜੀਉ ॥੨॥
akaal moorath ajoonee sanbha man simarath thandtaa thheevaan jeeo ||2||
of Undying Form, Unborn, Self-illumined: remembering You in meditation, my mind is filled with a deep and profound peace. ||2||
ਸਦਾ ਸੰਗੀ ਹਰਿ ਰੰਗ ਗੋਪਾਲਾ ॥
sadhaa sangee har rang gopaalaa ||
The Joyous Lord, the Sustainer of the World, is my constant Companion.
ਊਚ ਨੀਚ ਕਰੇ ਪ੍ਰਤਿਪਾਲਾ ॥
ooch neech karae prathipaalaa ||
He cherishes the high and the low.
ਨਾਮੁ ਰਸਾਇਣੁ ਮਨੁ ਤ੍ਰਿਪਤਾਇਣੁ ਗੁਰਮੁਖਿ ਅੰਮ੍ਰਿਤੁ ਪੀਵਾਂ ਜੀਉ ॥੩॥
naam rasaaein man thripathaaein guramukh anmrith peevaan jeeo ||3||
The Nectar of the Name satisfies my mind. As Gurmukh, I drink in the Ambrosial Nectar. ||3||
ਦੁਖਿ ਸੁਖਿ ਪਿਆਰੇ ਤੁਧੁ ਧਿਆਈ ॥
dhukh sukh piaarae thudhh dhhiaaee ||
In suffering and in comfort, I meditate on You, O Beloved.
ਏਹ ਸੁਮਤਿ ਗੁਰੂ ਤੇ ਪਾਈ ॥
eaeh sumath guroo thae paaee ||
I have obtained this sublime understanding from the Guru.
ਨਾਨਕ ਕੀ ਧਰ ਤੂੰਹੈ ਠਾਕੁਰ ਹਰਿ ਰੰਗਿ ਪਾਰਿ ਪਰੀਵਾਂ ਜੀਉ ॥੪॥੯॥੧੬॥
naanak kee dhhar thoonhai thaakur har rang paar pareevaan jeeo ||4||9||16||
You are Nanak's Support, O my Lord and Master; through Your Love, I swim across to the other side. ||4||9||16||
The Piaaraa, the Beloved is Unfathomable. He is profoundly deep, our infinite Lord of the universe. He is merciful and He is loving. In meditation, we discover his true nature. Fearless and free of hate. Immeasurable and Infinite. Unborn and Self-illumined. Guru Arjan is re-telling the Truth of the mool mantar in these words. This what we discover. And when this Truth is discovered, when it is truly embedded in our hearts, and understanding is obtained, we feel him as the Sustainer. He gives this priceless Treasure, Naam, which delivers us from pain and our minds are satisfied by the ambrosial nectar of the Naam.
The only thing left to say is that the words of these shabads in their original language are impeccable in their poetic beauty.