No, I mean literally falling.

This is a feat that's often tossed around as justification for the durability of baseline Space Marines, but oddly enough has never been properly calculated. So let's fix that, shall we?

The feat happens in the Horus Heresy Short Story, The Wolf of Ash and Fire, and here it is:

Horus dragged Ezekyle to his feet.
‘Can you fight?’ demanded Horus.
‘Aye,’snapped Ezekyle. ‘It’s just a scratch.’
‘Your shoulder is broken and the bone shield on your left side is fractured. As is your pelvis.’
‘They’d need to break every bone in my body keep me from your side,’said Ezekyle. ‘As it is for you and the Emperor, beloved by all.’
Horus nodded. To say more would be to shame Ezekyle.
‘No force in the galaxy will keep me from his side.’
As if Ezekyle’s words were a dare to the galaxy, Gorro convulsed in the grip of a violent quake that ripped up from far below.
‘What was that?’ asked Ezekyle.
There could be only one answer.
‘The gravitational fields keeping Gorro coherent are spinning out of control,’said Horus. ‘The scrapworld’s tearing itself apart.’
No sooner had Horus spoken than the deck plates buckled throughout the chamber. Metres-thick sheets of steel ripped like paper as geysers of oily steam belched from the depths. Bulging walls collapsed inwards and debris rained from the splintering ceiling. Cracking fissures spread across the bloody ground, tearing wider with every second as Custodians, Justaerin and orks fell into the scrapworld’s fiery depths.
Horus fought for balance, pushing to where he saw the golden light of the Emperor surrounded by greenskin marauders.
‘Father!’ yelled Horus.
The Emperor turned, one hand outstretched to Horus.
Another quake struck.
And the scrapworld swallowed the Emperor whole.
- The Wolf of Ash and Fire
Sejanus had no idea where they were. Everything was smoke and ash and blood. Three of his squad were dead already, and they hadn’t even laid eyes on the enemy. Red light painted the interior of the smoke-filled drop pod, dripping wet where Argeddan and Kadonnen’s bodies had been explosively gutted by spikes of penetrating debris. Feskan’s head rolled at his feet, leaving spirals of blood on the floor.
The drop pod’s boosters had failed and what should have been a controlled landing with the rest of the Fourth Company instead became a violent descent through hundreds of layers of honeycombed scrap towards Gorro’s core.
According to the squalling, static-filled sensorium on his visor, his company was around two hundred kilometres above him. The reek of scorched metal and rotten food poured in through tears in the side of the drop pod.
Sejanus heard the booming, clanking, screeching sound that was the hallmark of greenskin technology. And behind that, the guttural bark-language of orks. The sound had a grating metallic quality to it, but he didn’t have time to dwell on that now.
‘Up!’ he shouted. ‘Up now! Get out!’
His restraint harness endlessly ratcheted as the deformed metal tried to unlock. He wrenched it away and pushed himself upright, turning to rip his bolt pistol and sword from the stowage rack above. For good measure, he took a bandolier of grenades as well. The rest of his squad followed suit, freeing and arming themselves with complete calm.
- The Wolf of Ash and Fire

So a group of Space Marines fall down a height of two hundred kilometers, and survive with only minor injuries. Their Power Armours aren't damaged, and they quickly get up and continue fighting, which is really impressive.

To quantify how impressive this is, we need to know how much a Space Marine weights. Thankfully, we have official information regarding that:

Whilst wearing their power armour, an unarmed Space Marine typically stands slightly over 2.1m tall and weighs between 500-1,000 kg.
Source: Deathwatch Rulebook

Around 500 to 1,000 kg. This isn't accounting for Marines that are taller than 2.1m, nor for the weapons they carry, which are often too heavy for normal men to lift. So we can safely use 1,000 kg.

Plunging these numbers into the Splat Calculator, we get a final value of 1960000000.00 joules, or around 0.468 Tons of TNT.

Or Building level.

This scales to every Space Marine in the setting, and to every weapon which can damage Space Marine Power Armour.

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