From one end of the vast Plain to the other fear was the driving force. The fear came from two sources. There was the underlying knowledge that their one necessary resource was becoming scarce. There was enough to provide for all for the forseeable future but the extent of it was noticeably thinning. The degree of concern over this was not enough to engender panic in any way but it did spur a new endeavor. It was the second source of fear that drove itself home as more urgent. The fear of the Others and what appeared to be their imminent arrival. The Others had never been seen before but the ground vibrated with their presence. Fully half the population had been deployed in positions of defense, quietly waiting for the Others to break out onto the Plain.
The Plane was all the race knew of their planet's extent. The Plain provided everything they required. For countless generations the flat expanse was the only home they knew. As far as they were concerned it was the only Place. To the North and South were the ammonia seas. The opposite ends of the Plain, to the East and West, were bounded by extremely tall ridges stretching from sea to sea. Gradually becoming steep, and very ancient, the ridges were effective impassable barriers. They had never been climbed and traversed for the simple reason that, on their planet, evolution had never selected for the ability to climb. Survival depended on the ability to scrape the Plain's surface for the resource upon which life here depended...silicates. There was no reason to climb in order to do that nor, for that matter, was there anything to climb.
Silicate-based life was the only option for a globe with a solid surface almost entirely made up of these minerals. Tectosilicates were the most abundant. There was also a large percentage of single and double chain inosilicates as well as sorosilicates, cyclosilicates and orthosilicates. Combined with an atmosphere containing a high concentration of ammonia, life had found a way to establish itself here. These silicate beings literally scraped out their living. Simply scraping wherever they were when the need for sustenance arose ensured that the plain remained flat since no real patterns developed as they wandered about their realm. The problem began to become apparent when certain areas, widely separated at first, would become contaminated with ammonia seeping up from below. Once the contamination reached a certain level, the silicates there were no longer edible. These areas began to cover large swathes of the Plain and it was obvious that the whole Plain would someday become uninhabitable.
With a body that resembled a slightly bulged 3 foot diameter bottlecap they stood on eight legs. Each leg ended with a foot-like appendage shaped like the wide end of a paddle on a swivel-type joint. Their shuffling gait maintained a sharp edge all around each "foot". When swiveled perpendicular to the surface one or more "feet" easily scraped up a small pile of silicate powder for ingestion. Each of these beings would live for several thousand day/night cycles while budding one, sometimes two, offspring. In order to ensure an unspoiled life-giving Plain not littered with the graveyards of their deceased, all the dead were dutifully placed at the foot of either the West or East ridge. Time and the ever present particulate breeze would slowly abrade the remains. It was taken for granted that it was this practice, over the eons, that gave rise to the ridges in the first place. Death normally came by normal life-cycle functions that would be considered "old age". Accidental death was unheard of. There were no natural events or technological devices from which an accident might have sprung.
Violence among their race was not unheard of though. It wasn't commonplace but it wasn't an extreme rarity either. Each of these creatures was equipped with an acid injecting stinger that grew longer all throughout the normal lifespan. In this way, as one got older, one became more deadly. And deadlier in more than just one way. Just as with honey bees on Earth, the stinger was anchored to a vital organ in the abdomen. To inflict a sting meant death for both the stingee and the stinger. And just as with many humans on Earth, the older one became, the more easily one became aggravated and aggressive. Aggression was triggered when an area of the Plain became overly populous. Sporadic fights would erupt. Several individuals would get stung and the combatants would die while cooler heads would begin radiating to other areas of the Plain. This activity, along with their barely above replacement rate of reproduction, was responsible for a very slow growth of the entire population. As it was, the simple conclusion was arrived at that the older members representing half of the population would take up the defensive positions against an invasion from the Others.
The other half of the population was tasked with the only possible solution to their resource problem. They were all gathered along a width of the base of the Eastern ridge doing what they did best. Scraping. They had been at it for 3 generations now. Work began when it first was realized that ammonia seepage was becoming a problem. The plan was to scrape away a section of ridge and create an opening through which they could all migrate. Hopefully to a new area rich in silicates. Initially it was nearly the entire population along a much wider section of ridge. The gradual slope at the base was easily erased since that was very loosely built up. As time passed the width of area scraped funneled narrower and the large numbers participating at the onset could not be used efficiently. There were some overcrowding issues and too many fights were breaking out amid an endeavor meant to save all their lives. So many of the oldest members were sent off to live their lives on the Plain as had always been done before. The scraping continued and with each foot or so scraped to ground level, a little more of the edifice would tumble to the plain and be scraped away. In time the effort had progressed to the steeper, more solid portion of the ridge and a tunnel was the result. The tunnel was quite far along when first came the reports of the Others. Old ones arrived and made it clear that their Plain was soon to be inundated by an unknown intruding horde. The resource situation couldn't handle any large increase of consumption and the Plain must be defended. The decision was made to rally the older half together and send them across the Plain to protect what silicates were left. That was a full generation ago.
Once the defending forces had been set up, the scraping out of the tunnel took on a new vigor and greater sense of urgency. And then, one day, the wide tunnel's dead end fell away and light poured in. Immediately the tunnelers began to stream forward. They were instantly met with an unbelievably fierce horde of creatures like themselves brutally and efficiently stinging the newcomers. The horde was unstoppable as they charged into the tunnel to sting all that they came upon. Out the other side the horde charged and by day's end not a single tunneler was left unstung and not a single member of the horde had not inflicted a sting.
A mining probe forged ahead through the reaches of space just beyond its home solar system. As it passed near a small planetoid its auto-navigation system engaged. Three orbits around the roughly spherical body was enough for the following report sent back to base:
Planetoid---200 km diameter---surface: mostly liquid ammonia---atmosphere: high concentration ammonia---surface features: wide equatorial solid band of silicate/ammonia mix around entire body with single ridge on north/south axis---signs of life: none---capability to support life: none.
Pictures of the planetoid sent back clearly showed the north south ridge. A techician first receiving the images was thoroughly puzzled by how a single ridge like that could form on an otherwise dead and unremarkable body.