<p>There are several questions here, I'm only focusing on the question regarding sea level in the past as per the title question.</p>

<p>In short, global mean sea level (GMSL) has been both lower and higher than present in the past. As recently as the last glacial maximum around 25,000 years ago GMSL was around 120m below present as much of the water was held on land as ice. </p>

<p>However, GMSL doesn't really exist because sea level is not level, but is a useful way of summarising global trends. What people living in coastal areas experience is relative sea level (RSL), which is defined as the mean level of the sea surface relative to the level of the land. There are two key components affecting RSL: isostasy and eustasy. Put simply, isostasy refers to the vertical position or motion of the land (the Earth's solid crust sits on a molten mantle and is subject to vertical displacement from its equilibrium state governed by the balance of overburden pressures  i.e. a concentration of mass on land causes subsidence of the land beneath it whilst uplifting the land extending beyond the focus of the mass. Eustasy refers to a uniform, global change in sea level caused by fluctuations in the balance between water held on land (usually as ice) and in the oceans, or by a change in the capacity of ocean basin (usually tectonically related). RSL can vary considerably from GMSL (unsurprising since this is a mean measurement). For example (I'll use a British example here since that's where I'm from), at the last glacial maximum ice sheets covered the northern part of the British Isles from Scotland down to around the middle of England  (the exact southern extent is still debated in the scientific literature). The overburden of this ice effectively turned the British Isles into a giant see-saw with the land in northern part subsiding and the southern part uplifting. The effects of this are still felt today long after the ice has melted (and contributed a eustatic sea level component) as the land level readjusts to its equilibrium state( glacio-isostatic adjustment); the land in Scotland is rising and the land in the south of England is subsiding. So, whilst is true that globally sea level is rising  (at a rate of around  3.2 mm/yr), when combined with local land level changes then some places actually currently experience a sea level fall (in Scotland this is up to -1.5mm/yr) whilst in regions of isostaic subsidence the rate of sea level rise can be considerably greater than GMSL. </p>

<p>Since you mentioned gravity in your question I'll breifly introduce another curious component  (there are others,see reference below) which results in globally  non-uniform sea level changes. We are not experiencing a lower level of gravity  (gravity is a constant).  The amount of gravity that something possesses is proportional to its mass and distance between it and another object. On Earth mass is obviously not evenly distributed, with each location on the planet experiencing a different mass-dependant   perturbation of the gravitational field. So for example, as an ice sheet forms and grows  mass increases which results in a gravitational perturbation which attracts the ocen toawrds the ice sheet but as the ice sheet melts and the gravitational perturbation reduces that water that was previously drawn towards the ice mass is redistributed in distant locations from the ice mass. This (counterintuitively) means that the greatest rates of sea level rise as a consequence of Greenland melt are actually experienced in the Southern Hemisphere.</p>

<p>Hope this serves as a useful introduction. I would usually include more references but I'm writing this on my phone right now whilst travelling on a train. One key reference,which is written in a non-technical style, worth checking out is : 
Sea level is not level : The case for a new approach to predicting UK sea-level rise. / Gehrels, Willem Roland; Long, A.J.
In: Geography Compass, Vol. 93, No. 1, 01.03.2008, p. 11-16.</p>

CC BY-SA 3.0
2017-09-30T09:30:52.553Z
2017-09-30T16:17:34.180Z
2017-09-30T16:17:34.180Z
Tom Newton
<p>"Does that mean the earth's sea-level was much lower than it is now? If this is true, then are we experiencing a lower level of gravity?"\</p>

<p>There were indeed periods in the Earth's past when sea-level was was lower than what it is now. One of the main reasons for this is the Earth's glacial-interglacial cycles. During an ice-age, there are huge ice-sheets on the continents. The water for this ice comes from the oceans, thus the sea level gets lower when there is a lot of ice on the land. </p>

CC BY-SA 3.0
2017-10-02T15:13:53.773Z
2017-10-02T15:13:53.773Z
RRC
<p>A lot of times, ancient artifacts or even whole ancient civilizations are found buried very deep into the ground surface of the earth.</p>

<p>Entire civilizations that had high buildings have been found many meters below the earth.</p>

<p>Does that mean the earth's sea-level was much lower than it is now? If this is true, then are we experiencing a lower level of gravity?</p>

<p>How can just sedimentation of sand, bury big building to such depths?</p>

<p>If lack of habitation by humans eventually results in burying of things, then what about the places like Chernobyl, which has not been inhabited for a long time?</p>

<p>I am not a physics grad or anything, just a curious guy.</p>

<p>Any help is much appreciated.</p>

CC BY-SA 3.0
2017-09-26T09:44:39.917Z
You can read up on [past sea level](https://en.wikipedia.org/wiki/Past_sea_level) on Wikipedia.  It was certainly much lower in *prehistorical* times, such that people could walk to Britain and across the Bering Land Bridge, but that's rather 20–100k years ago.  It was not lower in the past 5k years.  Gravity is not related at all, what makes you think it would be?  Please split of your other questions; one question per post please!
gerrit
CC BY-SA 3.0
2017-09-30T17:32:42.767Z
Re Chernobyl, "a long time" in archeology is hundreds, if not thousands, of years, not a couple of decades.  And what makes you think the Chernobyl exclusion zone is not inhabited?  There are people living there, and IIRC ongoing, if intermittent, construction work to enclose the reactor debris.
jamesqf
CC BY-SA 3.0
2017-10-01T00:10:26.943Z
You have two rather distinct questions here, one about sea level and anther about burial I would recommend you split them into two questions.
John
<earth-history><sea-level>
Was the Earth's sea-level significantly lower in ancient times?
Bej
# Was the Earth's sea-level significantly lower in ancient times?

(Asked by: username_0 on 2017-09-26)

A lot of times, ancient artifacts or even whole ancient civilizations are found buried very deep into the ground surface of the earth.

Entire civilizations that had high buildings have been found many meters below the earth.

Does that mean the earth's sea-level was much lower than it is now? If this is true, then are we experiencing a lower level of gravity?

How can just sedimentation of sand, bury big building to such depths?

If lack of habitation by humans eventually results in burying of things, then what about the places like Chernobyl, which has not been inhabited for a long time?

I am not a physics grad or anything, just a curious guy.

Any help is much appreciated.

**Tags:** earth-history, sea-level

> **Comments:**
> **username_1**: You can read up on [past sea level](https://en.wikipedia.org/wiki/Past_sea_level) on Wikipedia.  It was certainly much lower in *prehistorical* times, such that people could walk to Britain and across the Bering Land Bridge, but that's rather 20–100k years ago.  It was not lower in the past 5k years.  Gravity is not related at all, what makes you think it would be?  Please split of your other questions; one question per post please!  
> **username_2**: Re Chernobyl, "a long time" in archeology is hundreds, if not thousands, of years, not a couple of decades.  And what makes you think the Chernobyl exclusion zone is not inhabited?  There are people living there, and IIRC ongoing, if intermittent, construction work to enclose the reactor debris.  
> **username_3**: You have two rather distinct questions here, one about sea level and anther about burial I would recommend you split them into two questions.  

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## 2 answers

### Answer by username_4 on 2017-09-30. Score: 7

There are several questions here, I'm only focusing on the question regarding sea level in the past as per the title question.

In short, global mean sea level (GMSL) has been both lower and higher than present in the past. As recently as the last glacial maximum around 25,000 years ago GMSL was around 120m below present as much of the water was held on land as ice.

However, GMSL doesn't really exist because sea level is not level, but is a useful way of summarising global trends. What people living in coastal areas experience is relative sea level (RSL), which is defined as the mean level of the sea surface relative to the level of the land. There are two key components affecting RSL: isostasy and eustasy. Put simply, isostasy refers to the vertical position or motion of the land (the Earth's solid crust sits on a molten mantle and is subject to vertical displacement from its equilibrium state governed by the balance of overburden pressures i.e. a concentration of mass on land causes subsidence of the land beneath it whilst uplifting the land extending beyond the focus of the mass. Eustasy refers to a uniform, global change in sea level caused by fluctuations in the balance between water held on land (usually as ice) and in the oceans, or by a change in the capacity of ocean basin (usually tectonically related). RSL can vary considerably from GMSL (unsurprising since this is a mean measurement). For example (I'll use a British example here since that's where I'm from), at the last glacial maximum ice sheets covered the northern part of the British Isles from Scotland down to around the middle of England (the exact southern extent is still debated in the scientific literature). The overburden of this ice effectively turned the British Isles into a giant see-saw with the land in northern part subsiding and the southern part uplifting. The effects of this are still felt today long after the ice has melted (and contributed a eustatic sea level component) as the land level readjusts to its equilibrium state( glacio-isostatic adjustment); the land in Scotland is rising and the land in the south of England is subsiding. So, whilst is true that globally sea level is rising (at a rate of around 3.2 mm/yr), when combined with local land level changes then some places actually currently experience a sea level fall (in Scotland this is up to -1.5mm/yr) whilst in regions of isostaic subsidence the rate of sea level rise can be considerably greater than GMSL.

Since you mentioned gravity in your question I'll breifly introduce another curious component (there are others,see reference below) which results in globally non-uniform sea level changes. We are not experiencing a lower level of gravity (gravity is a constant). The amount of gravity that something possesses is proportional to its mass and distance between it and another object. On Earth mass is obviously not evenly distributed, with each location on the planet experiencing a different mass-dependant perturbation of the gravitational field. So for example, as an ice sheet forms and grows mass increases which results in a gravitational perturbation which attracts the ocen toawrds the ice sheet but as the ice sheet melts and the gravitational perturbation reduces that water that was previously drawn towards the ice mass is redistributed in distant locations from the ice mass. This (counterintuitively) means that the greatest rates of sea level rise as a consequence of Greenland melt are actually experienced in the Southern Hemisphere.

Hope this serves as a useful introduction. I would usually include more references but I'm writing this on my phone right now whilst travelling on a train. One key reference,which is written in a non-technical style, worth checking out is :
Sea level is not level : The case for a new approach to predicting UK sea-level rise. / Gehrels, Willem Roland; Long, A.J.
In: Geography Compass, Vol. 93, No. 1, 01.03.2008, p. 11-16.


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### Answer by username_5 on 2017-10-02. Score: 2

"Does that mean the earth's sea-level was much lower than it is now? If this is true, then are we experiencing a lower level of gravity?"\

There were indeed periods in the Earth's past when sea-level was was lower than what it is now. One of the main reasons for this is the Earth's glacial-interglacial cycles. During an ice-age, there are huge ice-sheets on the continents. The water for this ice comes from the oceans, thus the sea level gets lower when there is a lot of ice on the land.


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