Unsuccessful Successful Terraformation

Terraforming.
A sweet dream of many science or science-fiction enthusiasts.
It became more than a dream long ago, with steady but gradual steps.
We first managed to revitalize barren lands, and wilted green spots.
From there on, the technology for terraforming just…snowballed.
We developed little capsules that released bacteria, chemicals, gases and various other elements that started the terraforming process.
After a few weeks, we would send explosives that were more showy than damage dealing to release huge amounts of energy, to fasten the process.
At first we tested it on asteroids, and in laboratory environments.
Obviously no proper, stable long standing atmosphere has been created due to the size of the tested objects, but overall the processes have headed into the  estimated directions.
Then our sights befell onto a target we’ve been eyeing for a long time: Mars.
We sent the little capsules, then we sent the special explosives.
We watched almost in real time how Mars, the red planet, became…purple.
Well, a shade of purple that represented vegetation, and a jade green that represented water.
We sent probes, and the results were magnificent.
We managed to reproduce the atmosphere almost perfectly like it is on Earth.
Well, it’s breathable, not toxic, will take some filtering but it’s not deadly anymore.
Obviously, Mars wasn’t a sterile planet, so the native bacteria, chemicals and materials in general reacted and combined with the terraforming material.
All these interaction were taken into consideration, simulated and predicted to a relatively high degree.
After all, we did have samples from the Red Planet.
But…it was…it was too successful.
Let’s start with the beginning.
The goal of this terraformation was simple.
More resources, more space to inhabit, to use as farm land, to use to grow animals and so on…
It’s not hard to see how useful an extra planet could be.
But, we miscalculated a tiny little bit about the conditions of the planet.
We never actually dug too deep.
We knew Mars still had waters underneath the surface, we knew that there is a high possibility that there might be life forms in the underground of the planet…
But we always thought it would be microscopic life form, or at best insect, bat or some rodent type of animals, nothing too big, as there shouldn’t have been enough resources for big animals…
We were wrong, so wrong.
Whatever was underground, the samples did no justice to that system.
It took the process around 30 years to finish, and 30 more for us to prepare.
3 month to arrive there…
And 3 days for 2 ships worth of crew members, mostly military, and architect, to be sadly lost…
The plants have a strange taste for blood.
The insects can chew metal.
And the animals…
They come out at night…
They’ve reaped hundreds of lives, and 2 ships, and yet we’ve no evidence of their existence…
So, now nobody dares to move to the new planet…
It was labeled as the “unsuccessful successful terraformation”, as the process itself was successful but it still didn’t satisfy our needs…
As the information is lacking, we can’t randomly send military personnel there, as who knows what terrors we’ve created…yet again…